From external reflectance infrared spectra directly obtained at the air-water interface, the conformation of poly(beta-benzyl L-aspartate) is established in monolayer films. It was found that a right-handed alpha-helix, 4.0(13) omega-helix, and a left-handed alpha-helix can all exist simultaneously at such an interface. The right-handed alpha-helix forms immediately when the film is cast on a surface of pure water, and these helices persist at all compressions in a Langmuir trough. An alternate conformation similar to the 4.0(13) omega-helix forms on addition of small amounts of 2-propanol to the aqueous subphase and converts either to left-handed or right-handed alpha-helices depending on the amount of 2-propanol added.