Mass spectral fragmentations of some s-triazine herbicides have been investigated using tandem mass spectrometry. Major pathways driven by the side chains have been confirmed. However, most of the previously proposed pathways which were thought to be characteristic of the ring have been found unlikely. A class of ring cleavage reactions characteristic of s-triazine rings with alkylamino side chains has been found and the mechanism has been proposed. In addition, tandem mass spectrometry has been utilized to differentiate tautomeric structures and to analyze the fragmentation reactions occurring from the mixture of isobaric ions.