The bubble formation properties of various porous surfaces at low gas supersaturations were investigated. Polystyrene and silica particles with pore sizes ranging from 40 to 1000 Angstrom were used as representative surfaces for this study. Among the polystyrene particles, those with the smallest pores generated the greatest numbers of bubbles at gas supersaturations of 5 and 15 atm N-2. However, at supersaturations of 50 atm N-2 all three polystyrene particles generated massive numbers of bubbles, obscuring any differences attributable to pore size. The surface wettability of the silica particles was altered by chemically coating the surfaces. Three of the four different silica particles initiated significantly greater numbers of gas bubbles after being coated with a hydrophobic silane layer. Hydrostatic prepressurization of the hydrophobic silica particles dramatically decreased the number of bubbles generated, suggesting that the bubble-initiating capability of these particles was due to gas trapped by the surfaces. In sharp contrast, the porous polystyrene particles showed no change in bubble nucleation capability following exposure to hydrostatic pretreatments of up to 1000 atm for 30 min. This suggests that gas not only may be trapped in the main pores of these particles, but also may be dispersed in undefined micropores throughout the polystyrene structure which may resist collapse by hydrostatic pretreatments of the magnitudes and/or durations used in this study.