A vapour flow field in the condenser region of heat pipes or thermosyphons in the presence of noncondensable gas was investigated experimentally and analytically. The flow field disclosed two major features; the one is that a thick boundary layer of condensing vapour was formed near the cooled condenser wall, and the other, an interfacial layer was observed across the flow field which will divide a region of pure vapour from that of noncondensable gas. Heat transfer rates of the vapour on the condenser wall were measured, and their empirical relation was deduced to discuss fluid dynamic formality of such a flow field. This relation indicates that a dimensionless heat transfer rate Nu from vapour flows to the cooled wall is almost inversely proportional to square root of the amounts of noncondensable gas contained in the vessel, and roughly proportional to a vapour flow Reynolds number Re in the condenser. A method of boundary layer approximation of the flow field was found to be inappropriate to predict Nu's as well as to describe the condensing boundary layer near the condenser wall. A set of full transport equations compoased of three conservation equations and a binary diffusion of vapour-gas mixtures were presented to formulate such flow field features. Some numerical results utilizing these equations are also presented to demonstrate a successful simulation of the experimental results.