We studied the simultaneous determination of demeton-S-methyl, oxydemeton-methyl, and their oxide demeton-S-methylsulfone, in agricultural products by liquid chromatography coupled with mass spectrometry (LC-MS). The sample homogenized with antioxidants L-ascorbic acid and butylhydroxytoluene was extracted with acetone. An aliquot of the crude extract was reextracted with ethyl acetate by using an EXtrelut column. After hexane/acetonitrile partitioning lipid-rich samples such as cereals, the extract was cleaned up on a PSA column or tandem graphitized carbon/PSA column, and determined by ESI-SIM mode LC-MS. Average recoveries (n=5) of compounds from ten kinds of samples fortified at the analyte concentration of 0.05 microg/g were from 73.8% to 102.5%, and the relative standard deviations were <or=5.7%.