In this paper, the H-shift, Curtius rearrangement and intersystem crossing (ISC) processes of singlet methoxysulfonyl nitrene CH3OS(O)(2)N have been investigated theoretically without and with one water molecule to explore the role of water to play in the title reaction. The calculation results demonstrate that the introduction of one water molecule is effective in stabilizing the transition states and reducing the barrier heights of the reactions compared to the isolated reactant, and the formations of singlet HNSO2 + HCHO and CH3ON + SO2 are the most feasible product pathways both in the absence and presence of one water molecule. (C) 2018 Elsevier B.V. All rights reserved.