The mechanism of the Cannizzaro reaction has been studied using the MNDO SCF-MO method. A hydride transfer between the intermediate (1) to the aldehyde (2)(R =R¹=H) was calculated to proceed via a symmetrical transition state with a barrier of 72 kJ mol^–1. Alternative mechanisms involving a single-electron transfer between (1) and (2) are predicted to be less favourable, particularly for aliphatic aldehydes. A radical chain mechanism involving a single-electron-transfer step is proposed as an alternative. Mechanisms involving dianions are discussed, in which single-electron transfers are particularly favourable, and radicals such as H˙ should be formed at high pH. Primary hydrogen kinetic isotope effects are calculated for all the steps involving hydrogen transfers and the results discussed in terms of the symmetry and properties of the transition states.