Modern separation and characterization techniques were used as the principal tools in a research investigation designed to reveal the nature of the impurities that, if not carefully controlled, can cause color degradation of petroleum naphthalene. Substantially all of the hydrocarbon impurities present in a sample of petroleum naphthalene were identified. The presence of four indans, tetralin, five indenes and two dihydronaphthalenes was established. Accelerated ultraviolet exposure tests provided evidence that extremely pure naphthalene is quite color stable. Exposure tests on blends of pure naphthalene and hydrocarbons representative of those identified as impurities showed that methylindenes are uniquely deleterious to ultraviolet stability. Bromine index provides a measure of the concentration of these objectionable impurities that is adequate for quality control purposes.