Simultaneous heat and mass transfer under thermal laminar free convection to a vertical electrode was investigated. On the basis of Marchiano and Arvia's theory developed in steady state, the ac diffusion impedance for a circular or a rectangular vertical electrode was calculated as a function of frequency. For this calculation, 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. With 80 terms in the series, the two solutions were shown to overlap fairly well for 1.6 ⩽ ξ ⩽ 10. The overall impedance was then obtained for the different electrode geometries. For a circular electrode these theoretical predictions were successfully tested with different steady temperature differences, ΔT, between the electrode and the solution (0° ⩽ ΔT ⩽ 2.65°).