Three-dimensional (3D) image patterns of phase-separated poly(methyl methacrylate) (PMMA)/poly (styrene-ran-4-bromostyrene) (PS-Br) blends heated at 180 degrees C for 6, 8, and 10 h were observed by 3D NMR microscopy with. 31) X-ray microscopy in order to characterize the 3D structure of the polymer blends using a standard 3D reconstruction protocol program for obtaining the 3D digital array of the PS-Br/PMMA morphologies. The phase-separated structure of the polymer blends in several 10-mu m scales was reasonably characterized. The phase-separated structure of polymer blends by 3D NMR images was quantitatively consistent with that by 3D X-ray images. It can be said that 3D NMR microscopy is a very useful means for analyzing the 3D structure of phase-separated polymer blends as well as 3D X-ray images. (c) 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 470-475, 2007