The development of suitable methods for analysis of the alkyl phosphate metabolites of organophosphorus pesticides in urine has been a rather d i f f i cu l t and persistent problem. Currently available methodology (BRADWAY et al. 1977; LORES & BRADWAY 1977; SHAFIK et al. 1973) suffers from several shortcomings. The extraction of the compounds from urine is often incomplete and inconsistent. The reagent used in derivatization, diazopentane, and its precursor, a substituted nitrosoguanidine, present unacceptable hazards to the analyst, principally because of the demonstrated mutagenicity and/or carcinogenicity of the reagents. In addition, when dimethylphosphorothioate and diethylphosphorothioate are derivatized with diazoalkanes, they to give two isomeric products: a phosphorothionate and a phosphorothiolate (SHAFIK et al. 1970). I t is, therefore, uncertain that the ratio of the isomers is constant. The limit of detection is higher than would be the case i f only one product were formed.