In the past it has been customary to analyze problems concerned with predicting the transient behavior of the concentration in terms of the diffusion equation and, in particular, in terms of Fick's Law. In this paper, the transient behavior of the vapor volumetric concentration in a two-phase flow system with a change of phase is formulated in terms of kinematic waves, and expressed as a propagation equation. The derived void propagation equation predicts the void response to variations of 1) power density, 2) liquid inlet velocity, 3) system pressure, 4) thermodynamic nonequilibrium in the liquid and/or in the vapor, 5) compressibilities of the liquid and of the vapor, 6) flow regime, and 7) body forces. For a boiling forced-flow system, the results of the analysis show that 1) The rate of propagation of the voids, as well as the change, i.e., the distortion of the void disturbance as it propagates along the duct, can be predicted by means of kinematic waves. 2) The rate of propagation of kinematic waves depends on the volumetric flux density of the mixture, the drift velocity of the vapor, and the vapor volumetric concentration. 3) Since the velocity of kinematic waves depends on concentration, the wave forms may develop discontinuities resulting in kinematic shock waves. 4) The void response depends, definitively, upon the flow regime and it may change, completely, with a change in flow pattern.