The causes for the persistence of a‐hexachlorocyclohexane in the atmospheric environment were explored. The isomerization of y‐hexachl orocycl ohexane to the a‐form is accomplished in solid state by photolysis under direct sunlight in the presence of ferrous‐ferric system during summer months. The isomer concentrations in the head space were measured before and after sunlight irradiation and after bringing the samples to a temperature of 21 ± 1°C. Under identical conditions, photochemical studies carried out with α‐, ß‐, γ‐ and δ‐hexachl orocycl ohexane isomers have unequivocally established that the a‐isomer is the most stable conformer photochemicaliy although the p‐isomer is known to be the prevalent metabolic product in mammalian species. Transformation of the HCH isomers to the stable α‐form may be one of the factors responsible for the presence of higher levels of the a‐isomer than the y‐form in the rain waters previously observed.