Mass transfer has been studied in gas-liquid stirred vessels with horizontal interfaces which appeared to the eye to be completely smooth. Special attention has been paid to the influence of the coefficient of molecular diffusion. The results are compared with those published before. The simplifying assumptions of identical hydrodynamical conditions at the same stirrer speed in one particular geometry, which have been made in some previous investigations, is shown to be wrong and may lead to incorrect conclusions on the influence of the diffusion coefficient. For the gas phase the mass transfer can be described by the penetration theory (Higbie, R., 1935, Trans. Am. Inst. Chem. Engrs35, 36–60) or surface renewal model (Danckwerts, P. V., 1951, Ind. Engng Chem.43, 1460–1467). With the use of a dimensionless equation, Sh, Re and Sc numbers, all data, even experiments carried out at elevated pressures, could be well correlated. For the liquid phase the results indicate that the mass transfer cannot be described by a simple model. The King model (King, C.J., 1976, Ind. Engng Chem. Fundam.5, 1–8), a combination of molecular and eddy diffusivity, is able to explain qualitatively the observed phenomena and the literature data.