Surface pressure-area per monomer (Pi-A) isotherms show that poly(L-lactic acid) (PLLA) Langmuir monolayers exhibit a liquid expanded-to-condensed (LE/LC) phase transition at low surface pressure. Brewster angle microscopy images show circular domains where the LC phase is surrounded by the LE phase during phase coexistence. Morphology studies via atomic force microscopy show that well-ordered patterns are only observed for Langmuir-Blodgett films prepared in the LC phase, while no ordered features are observed in the LE phase. The morphological differences confirm that during the LE/LC phase transition PLLA molecules form well-ordered structures at the air/water interface. Analysis by the two-dimensional Clausius-Clapeyron equation is used to predict the critical parameters (X-c). Both critical parameters, the critical temperature (T-c) and the critical pressure (Pi(c)), increase with increasing number average molar mass (M-n) as X-c = X-c,X-infinity - KMn-1, where X-c,X-infinity is the value of the critical parameter at infinite molar mass and K is a constant. For PLLA T-c,T-infinity = 36.2 +/- 0.3 degrees C and Pi(c,infinity) = 4.53 +/-0.06 mN.m(-1). This study provides a model polymer system for examining critical behavior in two dimensions.