Experiments on natural convection adjacent to horizontal plane surfaces were performed using the naphthalene sublimation technique. By analogy, the present mass transfer experiments correspond to heat transfer at a heated isothermal upward-facing plate or at a cooled downward-facing plate. Circular, square and 7:1 rectangular planforms were employed in the tests. Overall mass transfer coefficients and Sherwood numbers were determined and correlated as a function of the Rayleigh number. A common correlation for all three planforms was attained by using characteristic lengths equal to the ratio of the surface area to the perimeter. The transfer coefficients predicted by laminar boundary layer theory fell well below the experimental data and exhibited a different dependence on the Rayleigh number. The oft-quoted empirical correlation of Fishenden and Saunders was found to be a less satisfactory representation of the present data than that of Mikheyev. In the range of lower Rayleigh numbers, the results given by a numerical finite-difference solution were very much lower than the experimental data.