The deformation of a protein, myoglobin and two derivatives, apomyoglobin and peptide (1-55) has been studied at the air/water interface. The filled monolayer has been shown to have a thickness of ca. 16 Angstrom for the three materials, despite the different tertiary structures involved. It is proposed that the tertiary structure of the myoglobin and apomyoglobin is modified at the interface and it is the a-helical secondary structure which is the major determinant of the observed layer thickness. Additionally a small effect of the heme cofactor on the surface structure is identified for myoglobin not for apomyoglobin. The unfolding of the tertiary structure of myoglobin was greatest at the isoelectric point, coinciding with the largest surface excess. This unfolding seems to occur in the surface layer only, the partially filled second layer which forms at higher concentrations is about the same size as the solution protein dimensions.