The effect of broad spectrum ultraviolet (UV) light on γ‐hexachlorocyclohexane (HCH) in aqueous solutions with or without chlorine was investigated. γ‐HCH (lindane) was found to degrade mainly to hexane insoluble products more easily in the presence of chlorine than in the absence of chlorine. Further, the γ‐HCH isomer was found to isomerize to the α‐conformer whether chlorine was present or not. When an aqueous solution of lindane was irradiated with UV‐light for 105 hr in the absence of chlorine, about 44–45% of the residual sample was found to contain the α‐isomer. The presence of the α‐isomer was detected by gas chromatography and confirmed by gas chromatography‐mass spectrometry.