We investigate the miscibility of a polystyrene/poly(vinyl methyl ether) (PS/PVME) blend using nuclear magnetic resonance spectroscopy. We examine H-1 spin-lattice relaxation times in both the laboratory (T-1H) and rotating (T-1 rho H) frames at various temperatures. At temperatures lower than the glass transition temperature (T-g) of the blend, the observed H-1 relaxation time of PS is equal to that of PVME, showing that the 5/5 PS/PVME blend is miscible on a scale of 20-30 Angstrom. At temperatures much higher than T-g, the observed H-1 relaxation curve (T-1 rho H) of PS apparently differs from that of PVME. They are not single exponentials. The non-exponential decays are analysed taking into account spin diffusion; the H-1 spin diffusion rate between PS and PVME is found to be similar to 1000s(-1) at 38 degrees C. This spin diffusion rate is too slow for the T-1 rho H values of PS and PVME to coincide with each other. This is attributed to the fast molecular motion of PVME. The H-1 relaxation curve of the phase-separated blend formed by heating above the lower critical solution temperature is markedly different from that of the homogeneous blend. On the assumption that H-1 spin diffusion does not occur between phase-separated domains, we analyse the H-1 relaxation curve of each component polymer and obtain the stoichiometry of the phase-separated domains. We conclude that the phase separation of the 5/5 PS/PVME blend is initiated by spinodal decomposition; the phase separation rate is 0.5 min(-1) at 140 degrees C.