When a metal is electrodeposited in laminar flow onto a planar cathode facing upward in a channel, mass transfer rates are enhanced downstream beginning at some distance from the leading edge, because of the contribution of free convection to the fluid motion. The position and extent of this secondary flow has been clearly demonstrated by double beam interferometry, employed for the measurement of concentration profiles in the cathodic boundary layer. A narrow range of critical Rayleigh numbers, , has been associated with the onset of natural convection. This result provides a convenient basis for the prediction of the transition from forced convection control to combined forced and natural convection control of the mass transfer process.