diff --git a/src/SharpProp/Fluids/AbstractFluid.cs b/src/SharpProp/Fluids/AbstractFluid.cs
index 922b3d4..6731ec5 100644
--- a/src/SharpProp/Fluids/AbstractFluid.cs
+++ b/src/SharpProp/Fluids/AbstractFluid.cs
@@ -43,9 +43,17 @@ public virtual void Reset()
         _phase = null;
     }
 
-    public void SpecifyPhase(Phases phase) => Backend.SpecifyPhase(phase);
+    protected AbstractFluid SpecifyPhase(Phases phase)
+    {
+        Backend.SpecifyPhase(phase);
+        return this;
+    }
 
-    public void UnspecifyPhase() => Backend.UnspecifyPhase();
+    protected AbstractFluid UnspecifyPhase()
+    {
+        Backend.UnspecifyPhase();
+        return this;
+    }
 
     [SuppressMessage("ReSharper", "NonReadonlyMemberInGetHashCode")]
     public override int GetHashCode() =>
diff --git a/src/SharpProp/Fluids/Fluid.cs b/src/SharpProp/Fluids/Fluid.cs
index b1e0d8d..bde412d 100644
--- a/src/SharpProp/Fluids/Fluid.cs
+++ b/src/SharpProp/Fluids/Fluid.cs
@@ -45,6 +45,10 @@ fraction is not null
 
     public Ratio Fraction { get; }
 
+    public new IFluid SpecifyPhase(Phases phase) => (Fluid)base.SpecifyPhase(phase);
+
+    public new IFluid UnspecifyPhase() => (Fluid)base.UnspecifyPhase();
+
     public new IFluid WithState(
         IKeyedInput<Parameters> firstInput,
         IKeyedInput<Parameters> secondInput
diff --git a/src/SharpProp/Fluids/IAbstractFluid.cs b/src/SharpProp/Fluids/IAbstractFluid.cs
index aae11fa..0af7cb9 100644
--- a/src/SharpProp/Fluids/IAbstractFluid.cs
+++ b/src/SharpProp/Fluids/IAbstractFluid.cs
@@ -17,16 +17,4 @@ public interface IAbstractFluid : IFluidState, IDisposable
     ///     Resets all non-trivial properties.
     /// </summary>
     void Reset();
-
-    /// <summary>
-    ///     Specify the phase state for all further calculations.
-    /// </summary>
-    /// <param name="phase">Phase state.</param>
-    void SpecifyPhase(Phases phase);
-
-    /// <summary>
-    ///     Unspecify the phase state and
-    ///     go back to calculating it based on the inputs.
-    /// </summary>
-    void UnspecifyPhase();
 }
diff --git a/src/SharpProp/Fluids/IFluid.cs b/src/SharpProp/Fluids/IFluid.cs
index 1ff31ec..d164af6 100644
--- a/src/SharpProp/Fluids/IFluid.cs
+++ b/src/SharpProp/Fluids/IFluid.cs
@@ -21,6 +21,19 @@ public interface IFluid
     /// </summary>
     Ratio Fraction { get; }
 
+    /// <summary>
+    ///     Specify the phase state for all further calculations.
+    /// </summary>
+    /// <param name="phase">Phase state.</param>
+    /// <returns>Current fluid instance.</returns>
+    IFluid SpecifyPhase(Phases phase);
+
+    /// <summary>
+    ///     Unspecify the phase state and go back to calculating it based on the inputs.
+    /// </summary>
+    /// <returns>Current fluid instance.</returns>
+    IFluid UnspecifyPhase();
+
     /// <summary>
     ///     Returns a new fluid instance with a defined state.
     /// </summary>
diff --git a/src/SharpProp/Fluids/IMixture.cs b/src/SharpProp/Fluids/IMixture.cs
index 12727ec..a4475d3 100644
--- a/src/SharpProp/Fluids/IMixture.cs
+++ b/src/SharpProp/Fluids/IMixture.cs
@@ -20,6 +20,19 @@ public interface IMixture
     /// </summary>
     IReadOnlyList<Ratio> Fractions { get; }
 
+    /// <summary>
+    ///     Specify the phase state for all further calculations.
+    /// </summary>
+    /// <param name="phase">Phase state.</param>
+    /// <returns>Current mixture instance.</returns>
+    IMixture SpecifyPhase(Phases phase);
+
+    /// <summary>
+    ///     Unspecify the phase state and go back to calculating it based on the inputs.
+    /// </summary>
+    /// <returns>Current mixture instance.</returns>
+    IMixture UnspecifyPhase();
+
     /// <summary>
     ///     Returns a new mixture instance with a defined state.
     /// </summary>
diff --git a/src/SharpProp/Fluids/Mixture.cs b/src/SharpProp/Fluids/Mixture.cs
index fb67c42..44f1d46 100644
--- a/src/SharpProp/Fluids/Mixture.cs
+++ b/src/SharpProp/Fluids/Mixture.cs
@@ -66,6 +66,10 @@ public Mixture(IEnumerable<FluidsList> fluids, IEnumerable<Ratio> fractions)
 
     public IReadOnlyList<Ratio> Fractions { get; }
 
+    public new IMixture SpecifyPhase(Phases phase) => (Mixture)base.SpecifyPhase(phase);
+
+    public new IMixture UnspecifyPhase() => (Mixture)base.UnspecifyPhase();
+
     public new IMixture WithState(
         IKeyedInput<Parameters> firstInput,
         IKeyedInput<Parameters> secondInput
diff --git a/tests/SharpProp.Tests/Fluids/FluidTests.cs b/tests/SharpProp.Tests/Fluids/FluidTests.cs
index c215632..274f0d4 100644
--- a/tests/SharpProp.Tests/Fluids/FluidTests.cs
+++ b/tests/SharpProp.Tests/Fluids/FluidTests.cs
@@ -160,12 +160,11 @@ public void Reset_Always_ResetsAllNonTrivialProperties()
     [Fact]
     public void SpecifyPhase_Always_SpecifiesPhaseForAllFurtherCalculations()
     {
-        IAbstractFluid fluid = _fluid;
-        fluid.SpecifyPhase(Phases.Gas);
+        _fluid.SpecifyPhase(Phases.Gas);
         var action = () =>
-            fluid.Update(Input.Pressure(1.Atmospheres()), Input.Temperature(20.DegreesCelsius()));
+            _fluid.Update(Input.Pressure(1.Atmospheres()), Input.Temperature(20.DegreesCelsius()));
         action.Should().Throw<ApplicationException>();
-        fluid.UnspecifyPhase();
+        _fluid.UnspecifyPhase();
         action.Should().NotThrow();
     }
 
diff --git a/tests/SharpProp.Tests/Fluids/MixtureTests.cs b/tests/SharpProp.Tests/Fluids/MixtureTests.cs
index c7b731b..12b7926 100644
--- a/tests/SharpProp.Tests/Fluids/MixtureTests.cs
+++ b/tests/SharpProp.Tests/Fluids/MixtureTests.cs
@@ -46,6 +46,20 @@ public void Update_Always_InputsAreCached()
         mixture.Temperature.Kelvins.Should().Be(293.15);
     }
 
+    [Fact]
+    public void SpecifyPhase_Always_SpecifiesPhaseForAllFurtherCalculations()
+    {
+        _mixture.SpecifyPhase(Phases.Gas);
+        var action = () =>
+            _mixture.Update(
+                Input.Pressure(1.Atmospheres()),
+                Input.Temperature(20.DegreesCelsius())
+            );
+        action.Should().Throw<ApplicationException>();
+        _mixture.UnspecifyPhase();
+        action.Should().NotThrow();
+    }
+
     [Fact]
     public void WithState_VodkaInStandardConditions_PhaseIsLiquid() =>
         _mixture