The progress of homogenization in the mechanical blending of a butadiene-acrylonitrile copolymer (NBR) and a poly(vinyl chloride) (PVC) was observed with scanning and transmission electron microscopes. The significant events of the microscopic examinations were as follows : (i) NBR formed a continuous and PVC a dispersed phase. (ii) The skins of PVC grains (100-150 mu m) were removed first, and the grains were broken into agglomerates (similar to 10 mu m). (iii) The agglomerates were disintegrated stepwise into primary particles (similar to 1 mu m), domains (similar to 0.1 mu m), and eventually the nodular particles of the size between domains and microdomains (similar to 10 nm). The electron microscopic visualizations, together with the material behavior expressed in terms of the internal mixer geometry and the viscoelastic properties during mixing action, suggest a schematic model for the homogenization mechanism. The microphase structure of the finished blend was characterized by cross-polarization/magic angle spinning (CP/MAS) C-13 NMR spectroscopy. Analyses of T-1 rho spin-lattice relaxation for specific carbons permitted a precise identification of the microstructures. Double-component resolution of the magnetization decay confirmed a coexistence of a mixed phase and two microseparated phases, the latter corresponding to an unmixed NBR phase and PVC microcrystallites. From the T-1 rho relaxation times, the sizes of the microphases were estimated to be in the nanoscale.