We used scanning angle reflectometry to directly measure the extent of water-soluble polymer adsorption to insoluble dipalmitoylphosphatidylcholine monolayers at the air-water interface. Although high molecular weight polystyrene-b-poly(ethylene oxide) (PS-PEO) diblock copolymers are kinetically prevented from forming adsorbed brushes on solid surfaces, they did manage to adopt a more extended or brushy conformation at the fluid interfaces examined here. For both PS-PEO and PEO homopolymer, the ability of the adsorbing polymer to drive the Liquid expanded-to-liquid condensed monolayer phase transition facilitated monolayer penetration. The significance of this excluded area sink effect in the monolayer penetration mechanism depended on the polymer architecture. PS-PEO was a more effective monolayer penetrator than PEO homopolymer owing to its ability to increase the PEO block extension when penetrating denser monolayers. Thus, both polymer architecture and excluded area effects determine the effectiveness of monolayer penetration.