The rate of passage of molecules of carbon dioxide and nitrogen through the vapor–liquid interface of water at 300 K is studied by simulation. Previous work has established the form of the free energy profile which has a minimum when the solute molecule is on the surface and a barrier between this state and solution in the bulk liquid. In one set of simulations, trajectories were initiated in the gas phase. From these, the average lifetime of molecules in the surface is determined to be considerably longer than the inverse of the energy relaxation rate, so that the sticking coefficient is one and exiting molecules have no memory of their original velocities. However, most molecules do return to the gas phase rather than entering the bulk solution. The rate of passage of molecules over the free energy barrier is studied using the reactive flux method with trajectories initiated near the top of the barrier. The results for nitrogen, in particular, give a good plateau in the time-dependent transmission coefficient and hence a reliable rate constant. The results from these two sets of simulations are combined to give an effective interface width which is used to determine the permeability of thin water films. These results are compared to experimental permeabilities of thin Newton black soap films. The rate-determining step for solution in bulk water is not passage through the few Ångstroms width of the interface we study, but rather the transport from the vicinity of the interface into the bulk over the larger distance scale of μm.