The condensation zone of a condensing vapor flowing in a channel is idealized as an interface separating vapor and liquid phases of the fluid with a concentrated friction, heat removal, and cold liquid injector mechanism at the interface. Rankine‐Hugoniot relations are obtained including effects of the mass, momentum, and energy sources at the interface. For a stationary interface, an explicit approximate solution is obtained giving the final liquid state in terms of the other parameters. In addition, a rapidly converging iterative procedure is presented with this solution as a first approximation and utilizing directly thermodynamic tables of the vapor and liquid. Examples corresponding to steam flow in a pipe and to an underwater steam jet are considered and their Hugoniot curves are computed and compared to similar curves obtained in detonation theory. The thermodynamic requirements of the existence of a steam‐water interface limit these curves in extent and result in upper and lower bounds on the values of the cooling parameters.