Triblock copolymers of the form PEOalphaPPObetaPEOalpha [where PEO is poly(ethylene oxide) and PPO is poly(propylene oxide)] form stable insoluble monolayers at both the air-water and solid-water interfaces. At the air-water interface, the nominal equilibrium spreading area of copolymers that have the same PPO content, or "core", increases with increasing PEO content by an amount equivalent to the area of the hydrated ethylene ether subunits. With compression, a PEG-dependent transition from an expanded to a more condensed monolayer occurs at similar to 10 dyn.cm(-1), presumably because of condensation of PEO segments onto the core and out of the plane of the surface. Consistent with these results and interpretation, the thickness of adsorbed films of copolymers from series that have fixed cores increases approximately linearly with ethylene oxide content. At collapse of copolymer films, the nominal area per molecule-regardless of the number of ether functions in the PEO segment-is that of the hydrated PPO core. On the basis of these data, we propose models for the structure and ordering of triblock copolymers in condensed films, and we relate these models to the gradation in protein resistance observed for such films.