The mixed monolayer behavior of poly(methyl methacrylate) (PMMA) with different molecular weights and polystyrene-block-polyethylene oxide (PS-b-PEO) was investigated from the measurements of surface pressurearea per molecule (p-A) isotherms at three different temperatures (10 degrees C, 25 degrees C, and 40 degrees C). The miscibility and nonideality of the mixed monolayers were examined by calculating the excess surface area as a function of composition. The molecular weight of PMMA was demonstrated to have a major effect on its miscibility with PS-b-PEO. Negative or close to zero deviations from ideality of surface areas were observed for PMMA with the lowest molecular weight (12,000 g/mole). Mostly positive deviations were detected in mixed PMMA/PS-b-PEO monolayers with higher molecular weights of PMMA. Because of probable mushroom-to-brush transition effect, the miscibility between PMMA and PS-b-PEO was found to be the best at 25 degrees C among the experimental temperatures. (c) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012