The development of suitable methods for analysis of the alkyl phosphate metabolites of organophosphorus pesticides in urine has been a rather difficult and persistent problem. Currently available methodology 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. It is therefore, uncertain that the ratio of the isomers is constant. The limit of detection is higher than would be the case if only one product were formed.