The thermochemical parameters, vibrational frequencies, solvent isotope effects, and proton inventories for the neutral hydration of formaldehyde by water and by clusters containing two, three, and four water molecules have been calculated at 298 K for the gas phase, and also for water solvent, using abinitio molecular orbital theory at the MP2/6-31G* level and the self-consistent reaction field method. All of the stationary points required for an examination of a cyclic (cooperative) mechanism, first proposed by Eigen, have been found. Basis set superposition error has been taken into account, and this has allowed the calculation of the free energy changes associated with the different ways in which CH2O and (H2O)(n) (n = 1, 2, 3, 4, 8) can reach transition states containing different numbers of water molecules.