An improved and sensitive gas chromatographic method has been established for the determination of four dialkyl thiophosphates including dimethyl phosphorothioate, diethyl phosphorothioate, dimethyl phosphorodithioate and diethyl phosphorodithioate. The method is based on the derivatization of the thiophosphates with pentafluorobenzyl bromide, using hexadecyltrimethylammonium bromide as a phase transfer catalyst. The resulting derivatives were separated on an SE-54 capillary column and detected by an electron-capture detector. Several parameters affecting the derivatization of the related thiophosphates were investigated. Determinations of the thiophosphates were attainable at pmol levels. The detection limits (signal to noise ratio of 5) of dimethyl phosphorothioate, diethyl phosphorothioate, dimethyl phosphorodithioate and diethyl phosphorodithioate in aqueous sample were at about 8.3, 6.0, 5.7 and 5.6 nM, respectively. Application of the method to the analysis of the thiophosphates spiked in plasma, after simple ultrafiltration treatment, proved to be feasible.