Among the chlorinated aromatics, nitriles represent an increasingly important group of pesticides of which chlorothalonil (2,4,5,6-tetrachloroisophthalonitrile, I ) is a broad spectrum fungicide used against many plant pathogens affecting a large number of agricultural crops. Several studies on the photochemical transformation of I in different organic solvents, viz., benzene (Khan and Akhtar 1983 and references cited therein), dichloromethane (Dureja and Walia 1993), ethanol (Giumanini et al. 1989), methanol (Binkley et al. 1977 have been reported, but no attempt has been made to evaluate the toxicity of the transformed products. The present study has, therefore, been intended to re-investigate the transformation of I in alcohols (ethanol and methanol) under ultra violet irradiation for evaluation of the relative toxicity of the transformed products formed under the process. MATERIALS AND METHODSChlorothalonil was purified from commercial 'Kaboch' formulation (75% wettable powder) by dissolving it in chloroform and filtered. The concentrated filtrate was further purified by coloumn chromatography over silica gel and crystallised from petroleum ether-chloroform. Its purity (98.0%) was determined by TLC and HPLC (mp 250-251°C) and its authenticity was confirmed by IR,MS and NMR spectral data. Solvents used in the study were HPLC grade ( E Merck , India ) and all inorganic reagents were laboratory grade. Melting points were determined by an electrically heated mp apparatus and are uncorrected. Infrared spectra (IR) were determined on potassium bromide discs using a Perkin Elmer model 1310 spectrophotometer. 1 H and 13 C nuclear magnetic resonance (NMR) spectra were recorded at 100 MHz using a JEOL model JNM-FX 100 engine using tetramethylsilane (TMS) as the internal standard. Samples were dissolved in deuterochloroform. The mass spectra (MS) were obtained by direct sample probe on a JEOL Model JMS-DX 300 instrument at 70 ev.High pressure liquid chromatograph (HPLC) of Hewlett Packard Model 1050 equipped with a UV variable detector Model 1050 coupled with a HP 3392A integrator was used for the analysis of chlorothalonil and its metabolites. The reverse phase column (length 25 cm; id 4.6 cm; od 0.635 cm) was packed with Hypersil