Both physical surface rheology and computer simulation experiments were performed to understand the conformation of polymers in 2D systems. The interfacial stress rheometer was used to measure surface rheological properties of Langmuir monolayers of poly(tert-butyl methacrylate) with molecular weights between 81.5 and 780 kg mol(-1). The Pi-A isotherms for these monolayers show two transitions: a plateau at Pi approximate to 18 mN m(-1) and a bend at Pi approximate to 52 mN m(-1). The properties of the films are shown to be substantially different above and below the plateau surface pressure, Pi(p). Below Pi(p), the monolayers are primarily viscous, and surface viscosity increases linearly with MW, while above Pi(p), the films are more elastic, and surface viscosity is MW-independent. Computer simulations of these systems produce qualitatively similar Pi-A isotherms. The observed transition at Pi(p) marks the changeover from polymer chains existing in a single layer to forming regions of multilayers.