While the surface viscosity was at first greatly diminished, it subsequently recovered to values similar to those of the original aged gelatin film. The longer the film had been aged prior to addition of caseinate, the more rapid was the recovery. This behaviour can be interpreted in terms of the competitive displacement of the original primary adsorbed layer of gelatin by the more surface-active caseinate, followed by restructuring of the displaced hydrocolloid as a secondary gel-like layer below the new primary caseinate layer (as shown diagrammatically in the insets to Fig. 6). Though gelatin is unique in exhibiting thermoreversible self-association behaviour through triple-helix hydrogen bonding interactions, a qualitatively similar kind of protein–hydrocolloid organization at the oil–water interface can be expected with many other systems possessing favourable associative biopolymer–biopolymer interactions.