Spread monolayers of a cellulose alkyl ether (HPC-C16) have been investigated at the air/water interface by polarisation modulation infrared reflection-absorption spectroscopy (PM-IRRAS). Infrared spectra exhibit bands associated with the symmetric and anti-symmetric methylene stretching vibrations, as well as with the methylene scissoring vibration, at 2850, 2917 and 1471-1463 cm(-1), respectively. Air of these bands appear as positive peaks in the PM-IRRAS spectra, indicating that the hexadecane sidechains have a net orientation perpendicular to water surface. The relatively low frequencies of the methylene stretching bands suggest that the hydrocarbon sidechains adopt a near all-trans conformation. The intensity of these bands increases with decreasing molecular area during monolayer compression. This increase is due not only to an increase in the surface density of the absorbing groups but also to orientational changes. These results indicate that alkyl sidechain orientation at the air/water interface is, to a certain degree, dependent on the surface pressure. Furthermore, no change in orientation is observed during compression through the plateau typically present in surface pressure-area isotherms of these polymers.