The activity of bovine Cu,Zn superoxide dismutase (SOD) was determined by steady-state voltammetry using mercury microelectrodes having close hemisphere shapes. The measurements were carried out in aerated aqueous solutions saturated with the surfactant triphenylphosphine oxide (TPO). The influence of pH on oxygen reduction process, either in the presence or in the absence of SOD, was studied over the range 7.5-12.3. Well-defined steady-state voltammetric waves, whose heights were reproducible within 2%, were obtained. A theoretical equation was derived for the calculation of the kinetic constant involved in the SOD-catalysed dismutation of the superoxide ion. The results obtained with the approach employed here agreed satisfactorily with those obtained by a dropping mercury electrode with a short dropping time.