In all previous publications concerning the condensation of binary mixtures of vapours, the condensation rates are reported to be less than for pure mixtures. It is reported in this paper that for the condensation of weak binary vapours on a horizontal tube, where the vapour concentration of ammonia in steam is in the range 0.23-0.88 wt%, condensation heat transfer is enhanced by as much as 13%. This enhancement is due to the Marangoni effect which causes a 40% increase in heat transfer through the condensate film. This effect is moderated, however, by the increasing heat transfer resistance of the vapour film. The mechanism by which the heat transfer process is enhanced is an inherent feature of these weak binary mixtures and occurs over the entire condensing surface without promotion.