A numerical investigation is carried out for the phase change in the unsteady expansion of humid gases. The research is mainly focused on the process of water vapor non-equilibrium condensation in the presence of nitrogen and the influence of the condensation on the flow field in a shock tube. It is shown that the parameters of the flow field are disturbed and a condensation shock is generated due to the sudden release of the latent heat of condensation. With the condensation shock, the parameters and the structure of the flow field change dramatically.