We present a model describing flows which undergo a phase change caused by chemical reactions in the material flow. In mathematical terms, we consider a free boundary problem for the onedimensional, compressible Navier-Stokes system coupled with an Allen-Cahn equation. One free boundary arises at the boundary of the flow to a vacuum state with a jump in the density. Another approximate free boundary arises due to the Allen-Cahn equation modeling the phase transition of the flow. We consider a density-dependent viscosity µ = ρ θ and a density-dependent transition layer of thickness δ =δρ −1 . We establish the existence of a unique weak global solution, provided that θ < 1/3. 2010 Mathematics Subject Classification: 35Q30, 76Txx, 82B26.