Transmission electron microscopy and energy dispersive spectroscopy (TEM/EDS) were used to map the concentration profile at the interface of a poly(vinyl chloride) and poly(methyl methacrylate) bilayer. Thin sections in the order of 800 Angstrom were cut using an ultramicrotome, examined with TEM, and the concentration profile was mapped with EDS. The intensity of fluorescence X-rays was adjusted for sample thickness variation by measuring the relative thickness across the interface with electron energy-loss spectroscopy (EELS). The interfacial thickness of the bilayer after 6 h at 120 degrees C was 1.5 mu m, whereas the interdiffusion coefficient was determined as 8.0 X 10(-14) cm(2)/s.