A theoretical approach is presented to the problem of ohmic potential drops in response to potential-step excitation in circular thin-layer cells such as those used for in situ infrared reflection-absorption measurements of electrode surfaces. Both the faradaic current due to the reversible electrode reaction of adsorbed redox species on the electrode surface and the double-layer charging current have been taken into account. The problem formulated as an initial and boundary value problem was transformed into an integral equation, and then it was solved numerically. An approximate equation is presented which allows us to estimate the response time of the circular thin-layer cell from the relevant cell geometries and cell parameters.