Characteristics which control the existence of metastable (superheated) liquid states and the nucleation of the vapor phase have been studied; knowledge of these characteristics will assist in the prediction of cavitation in hydraulic equipment. The liquids were superheated by reducing the ullage pressure in a vessel of liquid which was initially in thermodynamic equilibrium. Experimental superheat limit curves for liquid nitrogen and liquid hydrogen, statically contained in a nearly ideal (acid cleaned, smooth glass) system, are presented and compared with theoretical limit curves. The effects of some nonideal liquid-solid interfaces on the nucleation characteristics have been studied and are also included. The theory developed in the text is in fair agreement with the experimental data for the nearly ideal system. Persistence of the metastable state is shown to be dependent upon the rate of pressure decay, the initial equilibrium pressure, fluid environment, and the nucleation pressure. Appreciable superheat persisted for long times with liquid nitrogen (e.g., 44.2 psi superheat persisted for 4.3 sec at 90 deg K) while the persistence times for liquid hydrogen were much shorter (e.g., 48.7 psi superheat persisted for only 0.030 sec at 26.7 deg K).