It is shown that, within a restricted potential region around the potential of zero charge at a dropping mercury microelectrode, the reduction of small particles of HgCl (+/- 0.3 mum), dispersed in a 0.1 M KCl + 0.9 M KF solution, can be observed in the form of current spikes. The potential dependence of the frequency of their occurrence is related to the structure of the electrical double layer at the mercury surface. Their distribution in time appears to be close to random. From the charge connected with one single spike, the size of the particle can be calculated, so that the method allows the size distribution of the colloid to be obtained. It is argued that, outside their faradaic region, the particles cannot be reduced, as a result of electrostatic repulsion.