The mechanisms and energetics governing the gas-phase reactions of a series of substituted singlet carbenes with water were Studied using highly correlated A initio molecular orbital calculations. Monosubstituted singlet carbenes ((1)[X-C-H]) were allowed to react with one and two water molecules in the gas phase (X = H, Me, CN, Cl, F for reactions with one water molecule and X = CN, Cl, F for reactions with two water molecules). Our results indicate the presence of stable ylide-like intermediates in all cases Studied, with overall and intrinsic barriers depending on the nature of the group bonded to the central carbon atom. For the reactions with one water molecule, it is found that, whereas all reaction profiles exhibit positive or near zero intrinsic barriers (intermediate -> TS), carbenes substituted with strong electron withdrawing groups (X = Cl, F) have positive overall barriers but carbenes bearing other substituents react in an overall barrierless fashion to produce the respective alcohols. For reactions with two water molecules, only the fluorine-substituted carbene exhibits an overall barrier. Classical transition-state theory with Eckart tunneling corrections (TST/Eckart) predicts the intermediate -> TS step to be about 3 to 6 orders of magnitude faster for the (1)[X-C-H] + 2H(2)O reactions than for the corresponding 1 water molecule cases. The competitive mechanisms and the effects of substituent and level of theory on the reaction paths are discussed in detail.