By means of the thin layer chromatography (TLC) a study was carried out on the decomposition of methyl parathion, ethyl parathion and sumithion when exposed to heat or ultra-violet irradiation. The results are briefly summarized as follows. 1. Parathions, when exposed to heat, form hydrolysates and such 0-analog as paraoxon as well as S-alky1 isomers. 2. When parathions are exposed to ultra-violet rays at 365 mμ and 254 mμ, the rate of decomposition is extremely slow. For example, when exposed to such rays in Petri dish for 5 hours, only a small amount of S-alkyl isomer is formed. 3. After heating parathions in a small test tube and conducting TLC, when each 0-analog and S-alkyl isomer above mentioned is confirmed, it is possible to identify a minute amount of each parathion by this method, and thus this method is feasible to apply to practical poison examination as a rapid and simple qualitative examination method.