The effect of progressive waves on a liquid surface upon gas absorption in unsteady state was investigated theoretically. The convective;diffusion equation for mass transfer under wavy flow was solved on the basis of a boundary layer approximation. An analytical solution for the dimensionless average mass transfer coefficient was obtained as a function of the dimensionless contact lime of gas and liquid tau and the wave velocity ratio U-c, which was defined by the ratio of wave velocity to the amplitude of surface velocity fluctuation. The solution is independent of U-c, and corresponds to that based on Higbie’s penetration theory for short contact time of gas and liquid. For long contact time of gas and liquid, it includes as special cases the analytical solution based on an eddy-cell model proposed by Theofanous et al. (1976) as well as an asymptotic solution in the limit of tau-->infinity obtained by Ruckenstein (1968).