Sets of adsorption isotherms of argon and krypton on the cleavage face of a number of lamellar halides have been determined. For each adsorbate-adsorbent pair a specific number of adlayers are formed during two-dimensional condensations, which are first order transitions. Particular attention has been paid to the entropy associated with these transitions. The number of adlayers that can form has been shown to depend essentially on the dimensional incompatibility between adsorbate and adsorbent