The mechanical properties of protein films at the gas-water interface are determined by means of a mobile barrier. The films are subjected to extensional and compressional surface stresses and strains as opposed to the commonly used shear ones. Two proteins, immunoglobulin G and albumin, with distinctly different characteristics at the interface are investigated. The results from albumin films demonstrate the validity of this method and indicate that, in certain cases, only one measurement is required to completely characterize the elastic constants of the monolayer. From the viscoelastic properties observed for IgG between surface pressures of 3.5 and 26 mN/m, possible mechanisms to explain the film’s behavior have been deduced.