Environmental context. Perchlorate, a well-known thyroid disruptor with both man-made and natural sources represents a major environmental problem in the United States but little information is available concerning the source of natural perchlorate. Previous research has demonstrated that perchlorate can be produced from exposure of some chlorine compounds to ultraviolet radiation, but no information was available how quickly or comparatively how much perchlorate was formed. The results of the present study can be used to evaluate the potential impact of ultraviolet processes on the overall occurrence of perchlorate in the environment. Abstract. The present study provides new and important information on perchlorate (ClO4-) formation through ultraviolet (UV) photodissociation of unbuffered chlorite (ClO 2-) solutions from the standpoint of kinetics under three different wavelength regimes having maximum emissions, e,max, at 235.7, 300 and 350 nm. ClO4- production rates and yields were in general found to be inversely related, with higher yields and lower rates at higher wavelengths, and vice versa. A simple kinetic model for ClO 4- production as a function of the ClO2 - first-order decay constant and starting concentration was fitted to the experimental data, resulting in the calculation of a rate constant, k 2, which is a function of light-source characteristics. Further, a conceptual scheme for ClO4- formation via photochemical reactions between oxychlorine species was proposed based on the experimental results and available literature. The present study is a further step towards understanding the formation of ClO4- from the photolysis of its precursors.