diff --git a/src/flow_coupler_types.jl b/src/flow_coupler_types.jl
index aeb66f1..2816f47 100644
--- a/src/flow_coupler_types.jl
+++ b/src/flow_coupler_types.jl
@@ -44,17 +44,21 @@ struct FlashedMixture2Phase{T, A<:AbstractVector{T}, E}
     critical_distance::Float64
     flash_cond::@NamedTuple{p::Float64, T::Float64, z::E}
     flash_stability::StabilityReport
-    function FlashedMixture2Phase(state::PhaseState2Phase, K::K_t, V::V_t, liquid, vapor;
-            vec_type = Vector{V_t},
+    function FlashedMixture2Phase(state::PhaseState2Phase, K::K_t, V::V_t,
+            liquid::FlashedPhase{V_t, A}, vapor::FlashedPhase{V_t, A};
+            vec_type = A,
             critical_distance = NaN,
             cond = missing,
             stability_report = StabilityReport()
-        ) where {V_t, K_t}
+        ) where {V_t, K_t, A}
+        # The phase vector type is part of the result type. Infer it from the
+        # phases instead of using the keyword value as a type parameter.
+        vec_type === A || throw(ArgumentError("vec_type must match the phase vectors"))
         if ismissing(cond)
             z0 = convert(K_t, fill(NaN, length(K)))
             cond = (p = NaN, T = NaN, z = z0)
         end
-        new{V_t, vec_type, K_t}(state, K, V, liquid, vapor, critical_distance, cond, stability_report)
+        new{V_t, A, K_t}(state, K, V, liquid, vapor, critical_distance, cond, stability_report)
     end
 end
 
diff --git a/test/runtests.jl b/test/runtests.jl
index 0c5c80e..b7c4334 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -235,6 +235,33 @@ using ForwardDiff
     @test dZ == 1.0
 end
 
+@testset "Static flashed mixture allocation" begin
+    function allocated_static_flash(V)
+        x = SVector(0.8*one(V), 0.2*one(V))
+        y = SVector(0.1*one(V), 0.9*one(V))
+        K = SVector(0.125, 4.5)
+        cond = (p = 1.0, T = 273.15, z = SVector(0.5, 0.5))
+        allocated_bytes = @allocated flashed = FlashedMixture2Phase(
+            MultiComponentFlash.two_phase_lv, K, V, x, y,
+            one(V), one(V), NaN, cond)
+        return allocated_bytes, flashed
+    end
+
+    # The flash is stored once per cell, so even a small per-result allocation
+    # produces substantial GC traffic on reservoir grids.
+    observed_bytes = Ref{Int}(0)
+    for _ in 1:2
+        ForwardDiff.derivative(0.4) do V
+            observed_bytes[], flashed = allocated_static_flash(V)
+            @test flashed.V == V
+            return flashed.V
+        end
+    end
+    if VERSION >= v"1.12"
+        @test observed_bytes[] == 0
+    end
+end
+
 @testset "Rachford-Rice derivatives" begin
     N = 25
     for z_light in range(0.0, 1.0, length = N)