The gas-phase reactions of the ion [CH3CHO/H2O](+.) have been investigated by mass spectrometry. The metastable ion (MI) mass spectrum reveals that this ion-molecule complex decomposes spontaneously by the losses of H2O, CO, and (CH3)-C-.. The structures of stable complexes and transition states involved in the potential energy surface (PES) have been studied by the G3//B3-LYP/6-31+G(d) computational method. Hydrogen- bridged water complexes have been found to be the major products of the losses of CO and (CH3)-C-.. The CO loss produces the [(CH3)-C-.center dot center dot center dot H3O+] ion and involves a "backside displacement" mechanism. The products corresponding to (CH3)-C-. loss have been assigned by theory to be [OC center dot center dot center dot H3O+] and [CO center dot center dot center dot H3O+], and their 298 K enthalpy values, calculated at the G3 level of theory, are Delta H-f-[OC center dot center dot center dot H3O+] = 420 kJ/mol and Delta H-f-[CO center dot center dot center dot H3O+] = 448 kJ/mol. The PES describing the interconversions among water-solvated CH3CHO+., CH3COH+., and CH2CHOH+. have been shown to involve proton-transport catalysis (PTC), catalyzed 1,2 H-transfer, and an uncatalyzed H-atom transfer mechanism, respectively.