The polystyrene/poly(vinyl methyl ether) (PS/PVME) blend is studied by ID and 2D Xe-129 n.m.r. The signal of Xe dissolved in miscible PS/PVME blends exhibits a single peak, and the chemical shift shows non-linear dependence on the PVME content. The observed dependence is explained by the decrease of the total volume of the blend with the increasing PVME content. In the blend heated above the lower critical solution temperature, the single Xe-129 peak splits into two peaks, which can be ascribed to the PS-rich and PVME-rich phases. The observed chemical shifts of a phase-separated blend provide the component ratios of each separated phase at the initial stage. Further, 2D exchange Xe-129 n.m.r. measurements for the phase-separated blends heat-treated for various periods are performed to determine the average domain sizes of the PS-rich and PVME-rich phases.