Voltammetry at Hg microelectrodes has been used to investigate the mechanism of the reduction of formaldehyde in conditions close to those employed for the electrosynthesis of ethylene glycol (that is, a very high concentration of formaldehyde in a neutral buffer at high temperature). It is shown that, even with a 40% formaldehyde solution, it is possible to record a well formed reduction wave, limiting current densities up to 30 A cm−2. The variations in half-wave potential, limiting current density and wave shape with formaldehyde concentration are reported and the influence of pH, temperature and electrolyte are considered. With increasing formaldehyde concentration, the transition from 2e− reduction to 1e− reduction is clearly observed. The results led to a re-examination of the influence of cathode material on the yield of ethylene glycol and it is confirmed that graphite is definitely the best choice.