Using X-ray diffraction (XRD) analysis, the porous structure of isotactic (it-) poly(methyl methacrylate) (PMMA) thin films fabricated with layer-by-layer (LbL) assembly of it-PMMA and syndiotactic (st-) poly(methacrylic acid) (PMAA) was observed for the first time. The XRD patterns of crystalline it-PMMA on a substrate changed when the surrounding st-PMAA eluted out and was incorporated. These results indicate that the polymer chains of it-PMMA moved at the solid-liquid interface.