An experimental study was made of diffusion behavior of methanol through three kinds of injection-molded plates of a polyoxymethylene (POM) copolymer with different molecular weights M at 60 degrees C. Fine structure of the three sample plates was also examined by wide-angle X-ray diffraction and small-angle X-ray scattering, and moreover, their dynamic properties were investigated by the dynamic mechanical analysis (DMA). It is shown that the diffusion behavior may be well explained by the one-dimensional Fick diffusion equation with a constant diffusion coefficient, and that the steady-state transport rate increases with increasing M. As for fine structure, the crystallinity decreases slightly, and the preferential orientation and the long period increase, with increasing M. The long period of the lamellar stacking structure increases with increasing M, and it also increases with methanol transport. In DMA, the loss tangent tan delta becomes higher after the methanol transport in the wide range of temperature around the glass transition one. These results indicate that amorphous regions serve as channels for methanol molecules in the lamellar stacking structure, leading to the conclusion that the dependence of the steady-state transport rate on M arises from the factors of crystallinity and long period. (c) 2007 Wiley Periodicals, Inc.