By Monte Carlo simulation the Knudsen layer in front of a surface from which atoms evaporate is studied. Evaporation into a vacuum is simulated by means of an evaporation–condensation geometry. Hard sphere interaction cross sections are employed. With the help of the present simulation data, the Knudsen layer is defined as that region adjacent to the evaporating surface, where the temperature of the flow parallel and perpendicular to the flow direction deviate by at least a given resolution &dgr;. Taking &dgr;=1%, it is found that the Knudsen layer is established after 800 mean‐free flight times; it has an extension of 20 mean‐free paths. It takes 60 monolayers to desorb before a Knudsen layer is formed. The data are generally in good agreement with predictions of analytical theory, where available. The differences observed in the case of evaporation from a binary target are discussed.