Natural convective mass transfer coefficients from vertical copper blocks with sinusoidal wavy surfaces of varying amplitude to wavelength ratios (a/lambda) have been experimentally obtained. A limiting diffusion current technique based on cathodic reduction of cupric ions was used. The mass transfer performance was observed to decrease with an increasing a/lambda. ratio. The average Sherwood number can be predicted for a wavy surface with given a/lambda by Sh = 0.915Ra(0.238)(1 + a/lambda)(-0.8577) within the studied range of Rayleigh numbers.