diff --git a/SharpProp.Tests/Fluids/FluidTests.cs b/SharpProp.Tests/Fluids/FluidTests.cs
index dda1014..00b99cf 100644
--- a/SharpProp.Tests/Fluids/FluidTests.cs
+++ b/SharpProp.Tests/Fluids/FluidTests.cs
@@ -172,6 +172,21 @@ public void Reset_Always_ResetsAllNonTrivialProperties()
             .WithMessage("Invalid or not defined state!");
     }
 
+    [Fact]
+    public void SpecifyPhase_Always_SpecifiesPhaseForAllFurtherCalculations()
+    {
+        IAbstractFluid fluid = _fluid;
+        fluid.SpecifyPhase(Phases.Gas);
+        var action = () =>
+            fluid.Update(
+                Input.Pressure(1.Atmospheres()),
+                Input.Temperature(20.DegreesCelsius())
+            );
+        action.Should().Throw<ApplicationException>();
+        fluid.UnspecifyPhase();
+        action.Should().NotThrow();
+    }
+
     [Fact]
     public void WithState_WaterInStandardConditions_PhaseIsLiquid() =>
         _fluid
diff --git a/SharpProp/CoolProp/AbstractState.cs b/SharpProp/CoolProp/AbstractState.cs
index 4e08a18..3d6aa4d 100644
--- a/SharpProp/CoolProp/AbstractState.cs
+++ b/SharpProp/CoolProp/AbstractState.cs
@@ -93,12 +93,24 @@ double secondInput
         SwigExceptions.ThrowPendingException();
     }
 
-    public virtual void Clear()
+    public void Clear()
     {
         AbstractStatePInvoke.Clear(_handle);
         SwigExceptions.ThrowPendingException();
     }
 
+    public void SpecifyPhase(Phases phase)
+    {
+        AbstractStatePInvoke.SpecifyPhase(_handle, (int)phase);
+        SwigExceptions.ThrowPendingException();
+    }
+
+    public void UnspecifyPhase()
+    {
+        AbstractStatePInvoke.UnspecifyPhase(_handle);
+        SwigExceptions.ThrowPendingException();
+    }
+
     public double KeyedOutput(Parameters key)
     {
         var result = AbstractStatePInvoke.KeyedOutput(_handle, (int)key);
diff --git a/SharpProp/CoolProp/AbstractStatePInvoke.cs b/SharpProp/CoolProp/AbstractStatePInvoke.cs
index 6491c9e..a67099a 100644
--- a/SharpProp/CoolProp/AbstractStatePInvoke.cs
+++ b/SharpProp/CoolProp/AbstractStatePInvoke.cs
@@ -45,6 +45,15 @@ double secondInput
     [DllImport(Library.Name, EntryPoint = "CSharp_AbstractState_clear")]
     public static extern bool Clear(HandleRef abstractState);
 
+    [DllImport(Library.Name, EntryPoint = "CSharp_AbstractState_specify_phase")]
+    public static extern void SpecifyPhase(HandleRef abstractState, int phase);
+
+    [DllImport(
+        Library.Name,
+        EntryPoint = "CSharp_AbstractState_unspecify_phase"
+    )]
+    public static extern void UnspecifyPhase(HandleRef abstractState);
+
     [DllImport(Library.Name, EntryPoint = "CSharp_AbstractState_keyed_output")]
     public static extern double KeyedOutput(HandleRef abstractState, int key);
 }
diff --git a/SharpProp/Fluids/AbstractFluid.cs b/SharpProp/Fluids/AbstractFluid.cs
index 4c81972..4c36fed 100644
--- a/SharpProp/Fluids/AbstractFluid.cs
+++ b/SharpProp/Fluids/AbstractFluid.cs
@@ -51,6 +51,10 @@ public virtual void Reset()
         _phase = null;
     }
 
+    public void SpecifyPhase(Phases phase) => Backend.SpecifyPhase(phase);
+
+    public void UnspecifyPhase() => Backend.UnspecifyPhase();
+
     [SuppressMessage("ReSharper", "NonReadonlyMemberInGetHashCode")]
     public override int GetHashCode() =>
         (
diff --git a/SharpProp/Fluids/IAbstractFluid.cs b/SharpProp/Fluids/IAbstractFluid.cs
index 136eb25..94426f9 100644
--- a/SharpProp/Fluids/IAbstractFluid.cs
+++ b/SharpProp/Fluids/IAbstractFluid.cs
@@ -22,4 +22,16 @@ IKeyedInput<Parameters> secondInput
     ///     Resets all non-trivial properties.
     /// </summary>
     public void Reset();
+
+    /// <summary>
+    ///     Specify the phase state for all further calculations.
+    /// </summary>
+    /// <param name="phase">Phase state.</param>
+    public void SpecifyPhase(Phases phase);
+
+    /// <summary>
+    ///     Unspecify the phase state and
+    ///     go back to calculating it based on the inputs.
+    /// </summary>
+    public void UnspecifyPhase();
 }