Mass transfer coefficients for regular packings determined for low flow velocities show to be influenced by natural convection. In order to apply the coupling rule for forced and natural convection, mass transfer correlations for both pure regimes are required. Since no relationship for natural convection could be found in the literature, a systematic study of free convection in regular packings was undertaken. General correlations were developed and it was shown, that they also apply to static mixers. A simple model was formulated which describes natural convection mass transfer in structured packings by correlations obtained For inclined surfaces. The mass transfer results measured for low Reynolds numbers could be correlated successfully by the coupling rule and also by a method using a modified Reynolds number.