The Langmuir monolayer of aequorin and apoaequorin was studied by infrared reflection-absorption spectroscopy (IRRAS) and polarization-modulated IRRAS techniques. The cc-helices in the aequorin Langmuir monolayer were parallel to the air-water interface at zero surface pressure. When the surface pressure increased to 15 mN(.)m(-1), the alpha-helices became tilted and the turns became parallel to the air-water interface. As for apoaequorin, the alpha-helices were also parallel to the air-water interface at 0 mN(.)m(-1). However, the cc-helix became tilted and the turns became parallel to the air-water interface quickly at 5 mN(.)m(-1). With further compression of the apoaequorin Langmuir monolayer, the orientation remained the same. The different behaviors of aequorin and apoaequorin at the air-water interface were explained by the fact that aequorin formed dimers at the air-water interface but apoaequorin was a monomer. It is more difficult for a dimer to be tilted by the compression of the Langmuir monolayer.