Detailed calculations are carried out for condensation of a binary zeotropic mixture outside a tube bank to investigate combined effects of mass transfer resistances in the liquid and vapour phases. The question is how a change in mass transfer resistance in the vapour phase can influence the heat flux. The surprising result is that a decrease in resistance in the vapour phase by reducing the tube pitch, and thus increasing the vapour velocity, reduces the heat flux between 20% and 50% for the conditions studied. This is due to a combination of mass transfer resistance, depletion of the heavy volatile component, and a reduction of the phase interface temperature. (C) 2004 Elsevier Ltd. All rights reserved.