The most important component of an absorption heat pump is the absorber in which vapor is absorbed into a liquid mixture. Commonly, the liquid mixture flows over tubes in a film mode. In this work, the condensation of a two-component vapor mixture into a two-component liquid film on a horizontal tube is considered. In general, the vapor is a volatile mixture and, theoretically, the mass transfer problem in the vapor must be solved concurrently with the problem in the liquid. In the present paper, the boundary condition on the mass fraction in the film at the interface is modified to account for this, and based on initial calculations of the dimensionless parameters; the solution for the vapor mass transfer problem need not be explicitly calculated. Analytical solutions are obtained for the mass fraction within the liquid film in terms of the pointwise absorbed mass flux. Numerical results are presented for an ammonia-water mixture typical of current applications. The case of a single-component vapor such as the case for a lithium-bromide-water mixture is also considered as a special case.