This report describes the condensation of steam, on a horizontal tube, when the steam contains a low concentration of trimethylamine. Previous tests in this programme have studied the addition of ammonia and methylamine; all of these tests were conducted at similar or identical operating conditions. The most significant difference in properties of these ammonia series additives is the magnitude of the surface tension variation that develops when their mass fractions are increased. All of the vapour additives generate Marangoni instabilities in the condensate film; the type of instability depends on the additive but at very low additive concentrations the rates of heat transfer through the film will be higher than those found when pure steam condenses. The morphology of the films formed during the condensation of steam, containing a low mass fraction of trimethylamine, are very different from those described in the earlier parts of this test series and the dynamic nature of this film morphology does not appear to have been previously reported. The video analysis of these films suggests that at low concentrations they form a very thin continuous film, which moves with a rapid circumferential velocity. At higher concentrations this film supports condensate drops that are rapidly formed at the top of the tube. These drops do not grow or move relative to each when they are conveyed down the tube and take time to be incorporated into the base layer at the bottom of the tube. It is not clear what influence these drops have on the rates of condensation. The size distribution of the drops tends to become more uniform at higher trimethylamine concentrations. Measurements of the drop velocities suggest that they and the base layer moves with a velocity of 0.1 m/s. The maximum heat transfer rate through these films is four times greater than that predicted by the Nusselt equation and it occurs with a trimethylamine mass fraction of 0.0003. (C) 2013 Elsevier Ltd. All rights reserved.