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 xi in the low-frequency range and proportional to xi(+1/2) in the high-frequency range. With 80 terms in the series, the two solutions were shown to overlap fairly well for 1.6 less than or equal to xi less than or equal to 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, Delta T, between the electrode and the solution (0 degrees less than or equal to Delta T less than or equal to 2.65 degrees).