Numerous experiments suggest that bubble formation in water is initiated by preexisting gas nuclei. This is unexpected since gas phases larger than the order of 1 μm in radius ought to rise to the surface of a standing liquid, whereas smaller ones should dissolve rapidly via the outward diffusion of gas that results from surface tension. Several mechanisms for stabilizing gas nuclei have been proposed, but in each case there is experimental evidence to the contrary. In this article, a model is investigated in which stability is maintained by surface‐active skins of varying gas permeability. Data on ultrasonic cavitation and on bubble formation by counter‐diffusion indicate that such skins, if they exist, must be initially permeable. Quantitative comparisons with bubble counts obtained recently from supersaturated gelatin lead to the further conclusion that nuclear skins become effectively impermeable if the static pressure is raised rapidly by a sufficiently large amount. The surface area, length, and energy per skin molecule deduced from these comparisons are remarkably similar to those of known surfactants, such as lung extract, lecithin, and oleyl alcohol.