The heat transfer rates that develop when ammonia water mixtures condense within a small, un-vented, horizontal, shell and tube condenser are examined. The vapor flow within the condenser was constrained by baffles and the condensate created formed a pool that flowed along the base of the condenser. During the test program the inlet ammonia vapor concentration to the condenser varied from 0 to 10 wt % and the maximum local vapor concentration measured was 26 wt %. The experimental results demonstrate that the condensation heat transfer rates generally decrease with increasing ammonia concentration, however at low ammonia concentrations (<2 wt %) the local and overall heat transfer rates for the condenser were enhanced. When the ammonia concentration was 0.9 wt %, the vapor heat transfer rate was 34 percent greater than that predicted by the Nusselt analysis for steam at the same conditions. This enhancement is attributed to the disturbed morphology of the condensate film, created by Marangoni instabilities.