An experimental and modelling study is reported of condensation in a cylindrical, two-phase reflux thermosyphon containing non-condensable gas. The diffusion equations in the interface region are solved numerically. The spatial distribution of condensing heat Aux is determined from accurate measurements of the thickness of the condensed liquid film. At high power levels, fluctuations can occur in the interface region indicating that, under such conditions, the vapour-gas interface is intrinsically unstable. At lower power levels, stable operation is observed and the width of the interface region is measured as significantly greater than that predicted by the diffusion model. The mass transfer in the interface region is strongly influenced by convection in this domain. At very low power levels, the experimental measurements of spatial distribution of heat flux in the interface region are in excellent agreement with the predictions of the diffusion model, indicating that diffusion is the dominant process which influences condensation in the interface region under these conditions.