To elucidate the role of protein conformation at the air/water interface, we measured the interfacial dilatational rheology of bovine serum albumin (BSA) and beta-casein adsorbed over long time periods using a modified dynamic pendant-drop tensiometer. Companion long-time dynamic surface pressure and ellipsometry measurements are also reported. BSA has well-characterized structural isomers (conformers) whose structural transitions depend on solution pH. It is, therefore, possible to establish a connection between protein structure and interfacial rheology without the ambiguity of comparing different proteins. We also studied beta-casein to verify the conclusions obtained from BSA. Adsorbed BSA and beta-casein protein layers are primarily elastic with the dilatational elastic modulus arising from two contributions: (1) conformational rearrangement following adsorption that leads to formation of an interconnected, sample-spanning interfacial protein network (i.e., an interfacial gel), and (2) the intrinsic structural stability of the individual protein units within the network. The latter component is most important to the interfacial dilatational modulus and explains why adsorbed layers of rigid, globular proteins are more elastic than those of flexible, random-coil proteins. We identify a new surface elasticity relaxation mechanism at the interface due to interprotein conformational rearrangement that is enhanced by electrostatic screening.