Organophosphorus (OP) pesticides are among the most widely used pesticides in the United States. Human exposure to these pesticides may occur from their use on crops in agriculture and for pest control in residential settings. Most of the OP pesticides used in the United States are metabolized to up to three of six common urinary dialkyl phosphate metabolites. Quantification of these metabolites provides information on cumulative exposure to most OP pesticides. To accurately quantify OP pesticide metabolites in human urine, we developed a simple, highly sensitive, analytic method involving automated solid-phase extraction (SPE) of human urine, followed by post-extraction derivatization of the organophosphorus metabolites with 1-chloro-3-iodopropane, and analysis by isotope dilution gas-chromatography-tandem mass spectrometry. The styrene-divinyl benzene polymer-based SPE cartridges yielded good SPE recoveries of the metabolites because of their enhanced non-polar interactions. This method is less labor-intensive, more time-efficient, and reproducible than previously reported methods. Automation of the SPE allowed unattended extraction of urine samples, and hence, increased the sample throughput and reduced the inter- and intra-day variations. The method limits of detection were excellent for all analytes ranging from 50 pg/ml to 170 pg/ml. Relative standard deviations ranged from 2% to 12%.