Aliphatic ethers of cellulose are found to form stable monolayers when spread from dilute chloroform solution at the air−water interface. Monolayer pressure−area isotherms exhibit a liquid analogous phase followed by a transition region of relatively high compression at constant pressure. Limiting molecular areas are found to depend on side-chain length, indicating that the hydrocarbon substituents do not adopt an orientation strictly perpendicular to the water surface. Monolayers of mixed ethers containing, on average, a single long chain substituent per repeat unit, form liquid analogous phases at molecular areas equivalent to the area of the anhydroglucose ring. It is thus concluded that the cellulose backbone lies flat on the water surface. The transition region observed upon compression beyond the liquid analogous phase is attributed to the formation of bi- or multilayers, with molecules leaving the water surface. This interpretation is consistent with the observed decrease in plateau pressure with increasing temperature.