A simple and sensitive method has been developed and validated for determining ethyl methylphosphonic acid (EMPA),isopropyl methylphosphonic acid (IMPA),isobutyl methylphosphonic acid (iBuMPA) and pinacolyl methylphosphonic acid (PMPA) in human urine using gas chromatography-tandem mass spectrometry (GC-MS/MS) coupled with solid phase derivatization (SPD).These four alkyl methylphosphonic acids (AMPAs) are specific metabolites and biomarkers of exposure to classic organophosphorus (OP) nerve agents VX,Sarin,RVX and Soman.The AM PAs in urine samples were directly derivatized with pentafluorobenzyl bromide on a solid support and then extracted by liquid-liquid extraction.The analytes were quantified with isotope-dilution by negatively chemical ionization (NCI) GC-MS/MS in a selected reaction monitoring (SRM) mode.This method is highly sensitive,with the limits of detection of 0.02 ng/mL for each compound in a 0.2 mL sample of human urine,and an excellent linearity from 0.1 t0 50 ng/mL.It is proved to be very suitable for the qualitative and quantitative analysis of degradation markers of OP nerve agents in biomedical samples.