An on-line automated method for photodegradation studies of isoproturon, diuron, atrazine, fenitrothion, and metoxuron by means of liquid chromatography/mass spectrometry (LC/MS) with particle beam (PB) interface is described. Surface water samples were first spiked with 50 µg/l of each pesticide and then exposed to the radiation of the medium-pressure mercury lamp. Next, in regular intervals of 60 min, aliquots of 50-ml sample were enriched on a solid-phase extraction (SPE) cartridge and eluted on-line by a gradient of LC eluent onto analytical column for separation, followed by MS detection. Photodegradation experiment was performed twice with each pesticide, with MS operated in electron ionization (EI) and positive chemical ionization (PCI) modes and ammonia as reagent gas. Additional spectral and quantitative information was obtained from ultraviolet diode-array detector (UV DAD) placed in-line between the outlet of the analytical column and MS detector. A great number of photodegradation products (DPs) of parent pesticides were detected and in numerous instances, tentatively identified. The appearance of several degradation products is reported for the first time.