Experiments were carried out to use thin-layer chromatography to determine pure substances of the organic phosphorous insecticides forat and phenothion. Three sorbents were employed to establish the most appropriate conditions for the application of a fast, readily applicably and highly sensitive method--silica gel G, DG and Kiselgur "Merk", as well as 14 mobile phases--monocompound (n-hexane, n-heptane, acetone benzol and toluol), double combinations (hexane-acetone 4:1, hexane-acetone 9:1, hexane-benzol 1:1, hexane-benzol 4:1, heptane-acetone 7:1, benzol-hexane 4:1, benzol-acetone 9:1, and petrolium ether-tetrachlormethane) and triple combinations (acetone-toluol-hexane 1:15 and 5 developers (a diazosalt, bromine vapours + diazosalt, palladium bichloride, bromphenol blue, and silver nitrate, sodium hydroxide, ammonia gas). Established was the fact that the best results are obtained with the use of silica gel G, the mobile phase heptan-acetone 7:1, and the developer of 0.2% solution of palladium bichloride in 0.5 per cent solution of hydrochloric acid. In chromatogrammes the forat compound appears as a tile red portion with a darker peripheral part and Rf = 0.61, and phenothion--in yellow, with a brownish halo and Rf = 0.41. The method is highly sensitive--for forat 0.5 microgram, and for phenothion 0.5 microgram. In determining these insecticides it is possible to use as a sorbent silica gel "Merk" 1:1 with the same degree of sensitivity, but with higher Rf values.