Surface pressure‐area isotherms of cellulose triacetate and amylose triacetate at the air‐water interface were determined on a vertical and horizontal film balance. Both polymers show anomalous behavior on the vertical balance, including: higher compressibilities than obtained on the horizontal balance, movement of the vertical plate, and atypical hysteresis of the isotherms. The behavior of the cellulose triacetate film is attributed to heterogeneous gelation of the monolayer at low pressures. A mechanism involving the lateral tilting of pyranose units at the interface and subsequent stabilization of the monolayer by van der Waal's interactions accounts for the properties of this film. A helical conformation for amylose triacetate at the air‐water interface is proposed to explain the low compressibility of this monolayer.

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