The steady-state flux of matter of molecular size from a mixture of vapor and noncondensable gas to the walls of a large containment vessel during the condensation of vapor can be predicted with the present analysis. A boundary layer approach has been used in formulating the mass-transfer problem and the resulting equations are solved numerically. Charts are presented for the flux of molecular iodine from a steam-air mixture to the walls of the containment vessel during the condensation of steam. Knowing the total pressure and the temperature of the bulk mixture, the wall temperature, and the concentration of air and iodine in the bulk mixture, the rate of removal of iodine from the vessel can be predicted. The analysis is correlated with an experiment and close agreement is found.