Polynuclear aromatic hydrocarbons (PAHs) are a ubiquitous and persistent group of toxicologically important compounds in the environment, thought to be susceptible to photodecomposition. A simple method has been developed for assessing the relative susceptibilities of compounds in solution to photodecomposition under simulated natural light. The method was applied to 11 PAHs, having a range of physicochemical properties. Measured half-lives varied by a factor,of 25, from < 2 h for anthracene to 50 h for fluoranthene. The results are discussed in the light of the factors and conditions existing in the natural aquatic environment. The procedure outlined in this paper produces results comparable to those obtained by more complicated methodologies.