Quantitative data concerning photoreactions of aromatic hydrocarbons were obtained in distilled water and several natural water samples. The reactions were studied at concentrations lower than the solubility limits of the aromatics, which in some cases were less than 1 microgram per liter. At these low concentrations, the photolyses were described by first-order rate equations, i.e., half-lives, were concentration-independent. Quantum yields and electronic absorption spectra were obtained for a series of naphthalenes, anthracenes, and polycyclic aromatics. These data were used in conjunction with simulated solar spectral irradiance and attenuation coefficients of water bodies to compute photolysis half-lives for the aromatic hydrocarbons in aquatic environments having varying depths and turbidities.