Behaviors of isotactic, atactic and syndiotactic poly(methyl methacrylate) (PMMA) monolayers at the air/water interface were investigated. The monolayer characteristics of the three stereoisomers were studied and compared in terms of surface pressure-area per molecule (pi-A) isotherm, hysteresis and relaxation phenomena, and the phase images observed from Brewster angle microscopy (BAM). The results show that iPMMA monolayer has a more extended and compressible property, and exhibits pronounced hysteresis and relaxation phenomena among the three tactic PMMAs. Such characteristics can reasonably be attributed to the higher flexibility of chains segments of iPMMA as estimated from its lower T-g. On the other hand, sPMMA, the one with the highest T-g, forms a more condensed monolayer due to the high cohesive interaction among polymer chains. The BAM images show that PMA4A molecules are not well extended on the subphase and thus, aggregative phases were observed even in a state corresponding to the gas phase. The aggregative structures are especially significant on sPMMA monolayer, which is consistent with the results estimated from the pi-A isotherms. When the temperature is elevated from 25 to 40 degrees C, all the monolayers of the three stereoisomers become more expanded and compressible due to the increase in chain flexibility. (c) 2005 Elsevier B.V All rights reserved.