Commercial polydisperse atactic poly(methyl methacrylate) (PMMA) exhibits a decreased glass transition temperature (T-g) when the film thickness is less than similar to 60 nm, whereas more model atactic PMMA shows an increased T-g in thin films supported on clean silicon wafers. NMR indicates no difference in tacticity, so the divergent thin film behavior appears related to the relative distribution of molecular mass. Extraction of some low molecular weight PMMA components from the commercial sample results in a significant modification of the thin film T-g compared with the initial PMMA fraction. The extracted sample exhibits initially a slight decrease in T-g as the film thickness is reduced below similar to 60 nm, but then T-g appears to increase for films thinner than 20 nm. These results illustrate the sensitivity of polymer thin film properties to low-molecular mass components and could explain some of the contradictory reports on the T-g of polymer thin films that exist in the literature. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2366-2370, 2010