Determining the aqueous solubilities and infinite dilution activity coefficients of polycyclic aromatic hydrocarbons (PAHs) is important for environmental reasons. However, common methods for the measurement of infinite dilution activity coefficients, such as ebulliometry or gas chromatography, cannot be used for solutes that are only very slightly soluble in water and have very low vapor pressures, such as the PAHs. Here we report values of the aqueous infinite dilution activity coefficients of four polycyclic aromatic hydrocarbons obtained by measuring their very low aqueous solubilities and their enthalpies of fusion. The values of the infinite dilution activity coefficients we obtain range from 4.2 x 10(5) for acenaphthene to 3.7 x 10(8) for benzo[a]pyrene, with an average error of +/-12.5%.