External and internal natural convection mass transfer rates were investigated at both vertical and horizontal cylindrical tubes using the electrochemical technique. Measurements were carried out with a. the tube outside surface only active (external flow) b. the tube inside surface only active (internal flow) c, both the tube outside and inside surface active (simultaneous external and internal flow) The electrode length and the electrolyte concentration were varied at constant temperature, External natural convection mass transfer rates were in good agreement with literature data, Internal natural convection mass transfer at vertical electrodes rates depended on the diameter-to-length ratio (d/L) and concentration. In the case of simultaneous external and internal natural convection, the measured mass transfer rate for internal convection was lower than that for external convection, Results were correlated in terms of d/L ratio, Sherwood, Schmidt and Grashof numbers.