This paper reports on the rates of condensation heat transfer for weak ammonia-water mixtures in a horizontal, shell and tube condenser. It is shown that for inlet ammonia concentrations in the range 0.2-0.9 wt.% the average condensation heat transfer for the condenser was enhanced by up to 14%. Furthermore, local enhancement of the condensation heat transfer of up to 34% occurred at local bulk vapour concentrations between 0.2 and 2 wt.% ammonia. This enhancement was caused by the Marangoni effect which produced a disturbed, turbulent banded condensate film and a corresponding drop in the thermal resistance of the condensate film. (C) 2004 Elsevier Ltd. All rights reserved.