Phase transformations and thermal stability of nickel-containing solid solutions of bismuth orthoniobate BiNbO4 were studied by methods of thermal and X-ray phase analysis. The DSC curve of the BiNbi(1-x).NixO4.delta samples exhibited endothermic effects near the temperatures of 1049 degrees C and 1227 degrees C during the heating, and exothermic effects at (similar to)990 degrees C during the cooling, which were associated with the phase transition from the orthorhombic to triclinic modification and with congruent melting. In the compacted polycrystalline samples of the solid solutions, the phase transition from the high-temperature triclinic modification to the orthorhombic modification was revealed after calcination at 750 degrees C for 150 h, as well as after 1 h calcination at 950 degrees C.