Rates of free convection mass transfer inside horizontal square ducts were measured by electrochemical technique. Physical properties of the solution and duct side length were changed to provide Sc.Gr range of 7.38 x 10(8) - 1.4 x 10(11). Under these conditions the data were correlated by the equation Sh = 0.382(Sc.Gr)(0.278) A comparison between the measured overall rate of mass transfer with that calculated by summing the rates of mass transfer at the separate sides of the duct showed that the measured value is higher than the calculated value owing to convective interaction between adjacent surfaces.