The solubilities of three aromatic hydrocarbons were determined at 25 C. in natural seawater and distilled water. Saturation was achieved by equilibration of water and an excess of hydrocarbon by mechanical agitation. All three hydrocarbons, toluene, acenaphthene, and pyrene, were less soluble in seawater than in distilled water. The magnitude of the 'salting out' effect increased with increasing molar volume of hydrocarbon. Temperature effects on aromatic-hydrocarbon solubility in seawater, studied at 15, 20, and 25 C, were also directly related to molar volume. Temperature, rather than salinity, variations during toxicological studies with aqueous solutions of aromatic hydrocarbons would appear to present a greater challenge to toxicant concentration stasis, particularly so for experiments utilizing concentrations near the solubility limit.