The photodegradatlon of femtrothlon was exammed m dlstdled water contauung methanol After 7 h of W Irradiation urlth a high-pressure mercury lamp, a fractlonatlon step was carried out on a senu-preparative sd~ca gel column Eighty-eight fractions were collected by increasing the polanty of an eluent mmture of n-hexane-ethyl acetate (95 + 5) up to 100% ethyl acetate Three more polar fractions m eluent nuxtures of n-hexane-acetone (50 + 501, pure acetone and acetone-methanol (25 + 75) were obtamed The breakdown products formed were ldentlfled by gas chromatography-mass spectrometly wth electron Impact (EI) lonlzatlon An estunate of the amount of femtrothlon degraded mdlcated that after 7 h of madlatlon, 10% of the parent compound still remamed and different oxldatlon, lsomeruatlon and solvolysrs products were produced The P = S group was oxldlzed to gwe femtrooxon and trunethyl phosphate m amounts of 0 1% and 3%, respectwely Carbomethoxyfemtrothlon, due to oxldatlon followed by solvolysis, and the lsomeruatlon products O,O,S-tnmethyl phosphorothloate and the S-methyl Isomer of femtrothlon were obtained as photoalteratron products at lo%, 10% and 3%, respectively Other compounds corresponded to demtrofemtrothlon, carbomethoxydemtrofemtrothlon, parathion-methyl and Cmtro-mcresol at 0 2%, 0 4%, 0 4% and 0 4%, respectively The El mass spectra and the structures of the Ions of the dtierent femtrothlon photolysls products are gwen