The effect of water on various neoprene compounds has been studied. Compounds cured with Pb3O4 are safer processing and are equal in water resistance to those cured with PbO. Conventional compounds cured with ZnO-MgO, while sufficiently resistant to water for most purposes, are inferior to Pb3O4-cured compounds. ZnO-MgO- cured compounds containing hydrated calcium silicate or fine particle silica differ from all others tested in that they exhibit equilibrium swell, and thus possess good physical properties even after extremely long immersions. Little difference was found in the water resistance of the various types of neoprene. Tests covering a range of temperature from 25 degrees C to 121 degrees C showed that maximum swell occurs at around 95 degrees C. Aqueous solutions cause less swelling than distilled water, the degree of swell decreasing with the concentration of the solution.