We modified the technique of scanning angle reflectometry to measure the surface excess concentration as water-soluble polystyrene-poly(ethylene oxide) (PS-PEO) diblock copolymers adsorb from solution to dipalmitoylphosphatidylcholine (DPPC) monolayers spread at the air-water interface. Polymers adsorb by penetrating the lipid monolayer. Surface pressure data and fluorescence microscopy indicate that PS-PEO adsorption drives the liquid expanded-to-liquid condensed phase transition in the DPPC monolayer by decreasing the available area per lipid. PS-PEO adsorption is therefore functionally equivalent to mechanical compression of the monolayer. Accordingly, the extent of PS-PEO adsorption accommodated by DPPC monolayers in different regimes of the surface pressure-area isotherm correlates with the compressibility of the penetrated monolayers. Polymer adsorption to Liquid expanded monolayers increases the interface compressibility to equal that of the phase transition regime. Polymer segments that adsorb to monolayers in the phase transition and the liquid expanded regimes force lipids from the expanded to the condensed state, thereby creating accessible interfacial area for the polymer. Polymer surface concentrations in those monolayer regimes are indistinguishable from those attained at the air-water interface in the absence of a spread monolayer. Liquid condensed monolayers cannot accommodate polymer segments in this way, and as a result, the extent of polymer adsorption is diminished.