The stability of the lead scandium niobate (PSN) and its solid solution with lead titanate (PSNT), both with the perovskite structure, was investigated in a platinum capsule under hydrothermal conditions. The perovskite solid solution of PSNT decomposes in water at 640 degrees C, which results in the transformation to the tetragonal structure after the treatment. In KOH solutions, the PSNT powder was partially dissolved, it results in the precipitation of Pb2ScNbO6 powders with the perovskite structure, and the growth of both lead oxide (massicot) and K4Ti3O8 needle crystals at the top of the capsule. In contrast, single crystals of both PSNT or PSN were grown in KF solutions at the top of the capsule by hydrothermal treatments at 600 degrees C with a temperature gradient of 40 degrees C. In both cases a deficiency of the scandium content achieves the stabilization of the pyrochlore structure of the grown crystals.