The fate of N-ethyl maleimide (NEM) on wastewater was monitored by liquid chromatography with ion trap mass spectrometry (LC-IT-MS). Activated sludge mixed liquor after exposing to NEM shock load was collected, centrifuged and filtered. Its soluble fraction was analyzed by LC/MS/MS and was found to contain a compound with m/z =144 in the positive mode and m/z = 142 in the negative mode. In addition, NEM shock load samples added with sodium azide showed an ions with m/z = 187. Fragmentation pattern using MS/MS showed that the formed product in NEM shocked samples is maleamic acid while those NEM shocked samples with azide is a glutathione conjugate. Samples from the synthetic wastewaters gave the same results.