The response of an electrode, initially at equilibrium, to an applied potential step, is modelled with an equivalent circuit. The model is valid for reactions that are slow in comparison with the time constant of the decay of charging currents. In the measurement, an external resistor is added in series with the working electrode to match approximately the charge transfer resistance, R-CT, of the electron transfer. Extraction of R-u, is then particularly simple. In addition to R-CT, the uncompensated resistance, R-u, and the double layer capacitance, C-dl, can be determined. An advantage of the model is that even if the potentiostat rise time is far from ideal, R-CT can be obtained reliably if R-u is known. Criteria for applicability of the equivalent circuit model are derived.