The exposure of 3i P-Leplophos on thin layers of silica gel to UV-and sunlight led to the degradation of the insecticide. The oxon, desmethylleptophos, desbromoleplophos, O-methyl phenylthiophosphonic acid and phenylphosphonic acid were identified. Leptophos was more readily degraded in chloroform than in benzene solution when exposed to UV, and the major degradation product was identified as desbromoleplophos. A smaller percentage of the oxon was also formed by V V-irradiation. When a solution of Leptophos in aqueous acetone leas exposed to sunlight and UV-irradiation, a variety of compounds including the oxon, O-methyl phenylthiophosphonic acid, phenyl phosphonic acid and a substance of unknown nature were obtained. On glass, pastic and quartz surfaces, Leptophos teas found to be relatively stable. On leaf surfaces, the oxon though formed in a small percentage represented the major transformation product of Leptophos.