AbstractEquations are derived to predict the effect of trace surface‐active impurities on mass transfer across a spherical interface.Transfer in the bulk phase surrounding spheres with both rapid and slow internal circulation, and moving in creeping flow, is shown to decrease on increasing the surfactant concentration.Drag and transfer equations for contaminated rapidly‐circulating spheres moving at high Reynolds numbers are in semi‐quantitative agreement with published data.The influence of surface‐active contaminants on drag and transfer decreases as the viscosity of the bulk phase is in