This paper reports the development of a fully automated method for the multianalyte determination of twenty pesticides belonging to different classes (triazines, phenylureas, organophosphates, anilines, acidic, propanil, and molinate) in natural and treated waters. The method, based on on-line solid-phase extraction–liquid chromatography–electrospray–tandem mass spectrometry, is highly sensitive (limits of detection between 0.004 and 2.8 ng L−1), precise (relative standard deviations between 2.0 and 12.1%), reliable (two selected reaction-monitoring transitions are monitored per compound), rapid (45 min per sample), and simple. The application of this method to the monitoring of the target compounds in a waterworks revealed the presence of the pesticides investigated at concentrations up to 516 and 82 ng L−1 (total pesticide concentration) in river water and ground water, respectively, used as sources, and their gradual removal through the purification process. [ABSTRACT FROM AUTHOR]