Solid-state decomposition of nearly equimolar ZrC–NbC solid solutions was studied by x-ray diffraction. The results demonstrate that annealing broadens the diffraction peaks from the solid solutions. Analysis of the peak width as a function of the diffraction angle indicates that the broadening is due to the decomposition of the solid solution into two coherent isostructural (B1) phases close in lattice parameter (0.4573 and 0.4561 nm) and the formation of nanometer-sized (~ .70 nm) grains. The nanostructure remains stable during long-term (100 to 700 h) annealing between 670 and 1270 K.