An ab initio Hartree-Fock and Möller-Plesset study of the radical cations obtained by ionization of the formaldehyde-water hydrogen bonded dimer is reported. Two radical-cation complexes with different bonding patterns were identified at the SCF level (2a and 2b), one of which (2a) closely resembles the formaldehyde radical cation (FRC) solvated by a water molecule. Geometry optimization at the MP2 level, however, demonstrates that 2a is not stable as a FRC-water complex. It undergoes proton transfer instead, to end up as a H3O+·HCO• complex, with the hydronium ion bound to the C atom of the radical. The reaction paths for this complex were investigated and compared with those of FRC. It was found that the most stable complex on these reaction paths is obtained after a H3O+ 1,2-shift in the FRC. This product, best described as a complex of HCO· and H3O+, is analogous to the recently reported HO•⋯H3O+ and HCO•⋯NH4+ and lies in a well deep enough to be observable experimentally. Different possible outcomes of the experiments are discussed in the light of the relative stability of the complexes obtained on the reaction paths. © 1993, American Chemical Society. All rights reserved.