Solidification of Y1.5Ba2Cu3Ox system was studied with the aim to find the temperature of self-nucleation. The microstructure after cooling from isothermal dwells in the temperature range 984-995 degrees C was analysed. It is shown that blocky YBa2Cu3O7-delta crystals were self-nucleated and grown during isothermal dwells below 988 degrees C and spheroidal multicrystals were formed during cooling from isothermal dwells at higher temperatures. The symmetrical and radial YBa2Cu3O7-delta crystal branches of spheroids grow in the < 110 > crystal direction. (C) 2011 Elsevier B.V. All rights reserved.