The thickness dependence of the glass transition temperature, T-g(h), has been investigated using ellipsometry at a variable temperature for thin films of partially miscible stereoregular poly(methyl methacrylate) (PMMA)-low-molecular-weight poly(ethylene oxide) (PEO) blends. These values were compared to T-g(h) values of pure stereoregular PMMA thin films, and the miscibility of PMMA/PEO systems is assessed by the so-called plasticizing effect of PEO, that is, DeltaT(g)*. In thin-film geometry, the miscibility of i-PMMA is enhanced compared with that in the bulk and becomes higher than that for s-PMMA in contrary to the bulk behavior of the blends. The conformation energy DeltaE of the PMMA chains in the thin film increase much more for s-PMMA than for i-PMMA, indicating larger gauche to trans conformational rearrangement for the former isomer in the confined geometry. These large local modifications of the s-PMMA conformation in the thin film could result in modification of the mixing entropy and, therefore, be the main reason for the lower miscibility of PEO in s-PMMA as compared to that in i-PMMA.