Experimental data are presented on the transient response of the vapor volumetric concentration in a boiling liquid. The experiments were conducted with Refrigerant-22 in forced upward flow through an electrically heated circular tube. The experiments were performed by oscillating the power input to the metal test section while maintaining a constant discharge pressure and a constant inlet liquid velocity. The amplitude of the power oscillations was varied between 5 and 40% of the average power at frequencies between 0.01 and 10 cps. Steady-state and transient void fractions were measured at six axial locations using a two-beam x-ray attenuation traverse method. Satisfactory agreement is shown between the results predicted by the void propagation equation and the experimental data. It is shown in particular that: 1) The rate of propagation of the voids as well as the wave form of the void disturbance as it propagates along the cluct can be predicted by means of kinematic waves 2) The void response depends upon the flow regime 3) Both the propagation velocity and the wave form can be predicted rather accurately if the effects of flow regime are taken into account. The loss of accuracy, introduced by not considering the effect of flow regime, is also demonstrated.