The ac diffusion impedance at small electrodes has been calculated as a function of frequency for convective‐diffusion conditions. The dimensionless local flux was expressed as a series expansion of the dimensionless local characteristic frequency ξ in the low‐frequency range, and proportional to in the high‐frequency range. The two solutions were shown to overlap fairly well for . The overall impedance was then obtained for a small circular or a rectangular electrode. These theoretical predictions were successfully tested in pipe flow for laminar or turbulent conditions.