The vaporization of an entrapped mass of saturated water suddenly allowed to expand is studied both experimentally and theoretically. In the experimental set-up, the water is initially pressurized by a piston at the closed end of a cylinder and heated to the onset of vaporization, at which time the expansion is initiated by releasing the gas counterpressure acting on the back face of the piston. The initial depressurization rate is mainly controlled by the gas pressure releasing mechanism, which, in the present study, consists of either a fast opening electric valve or a diaphragm breaker. Pressure measurements along the path of the piston indicate a significant pressure undershoot when compared to the numerical finite element predictions based on a one-dimensional, homogeneous equilibrium flow. Pressure undershoots up to 290 kPa for a initial water pressure of 600 kPa have been measured and are caused by non-equilibrium vaporization delays which affect the whole expansion range tested (volume ratio up to 50). For the sake of comparison, reference is being made to the Simpson and Silver non-equilibrium prediction model. The vaporization process is illustrated by a high-speed movie taken through a transparent cylinder. A sharp separation front appears between the steam and water phases as the light high quality steam migrates faster towards the face of the moving piston.