The quantum properties of a Bose‐Einstein gas give rise to a critical adsorption into a mobile film. If the energy of adsorption is assumed to decrease with thickness according to a simple power law, the theory gives an isotherm in good agreement with observations on helium adsorption at temperatures above the lambda‐temperature of liquid helium, for all gas pressures higher than 10 percent saturation.With mixtures of He3and He4isotopes the theory predicts appreciable adsorption of He3only in the first or localized monolayer; all mobile monolayers should be practically pure He4at all gas pressures. Comparison is suggested with the known isotopic constitution of the Rollin film below the lambda‐temperature.
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