Abstract: Liquid chromatography-mass spectrometry (LC-MS) with a particle beam (PB) interface is used to separate and identify a group of pesticides. The mass spectra obtained under the different ionization modes, electron ionization (EI) and positive and negative chemical ionization (PCI and NCI) are compared. The operating conditions under each mode, determined by studying the influence on the ion abundance of the ion source temperature of the EI mode, and the gas pressure and ion source temperature in the methane CI were optimized. EI was more sensitive than PCI and NCI and of the latter two modes, NCI gave higher responses, especially for organophosphorus compounds. When online solid-phase extraction-LC-PB-MS was applied to real samples, limits of detection in full scan mode were in the range of 0.5 and 10 µg L-1 for EI. The analysis of real samples by online solid-phase extraction-LC-PB-MS enabled EI detection of one of the pesticides studied and confirmation by PCI and NCI. The combined EI/CI information also enabled the detection of some non-target compounds.