The quaternary ammonium compounds, diquat and paraquat, are commonly used herbicides. This method is used for the determination of diquat and paraquat in water samples by solid-phase extraction and liquid chromatography/diode array detector. And the molecular ion and characteristic mass ions of two compounds are confirmed by using positive ESI/MS. The concentrations and species of the ion pair reagents, column temperature, the pH and the species of the extracting sorbents were optimized to get high recoveries and good chromatographic resolution between diquat and paraquat. Extraction recoveries of diquat and paraquat were 89% and 93%, respectively by using C18 cartrige at pH 11.5 of sample and 2㎖ of solvent volume, and the relative standard deviations were less than 5%. The mobile phase was adjusted to 0.05M of 1-heptanesulfonic acid at pH 5.5 and column temperature at 40℃ for baseline separation of them.