The growth of YBCO bulk superconductors with Y1.5Ba2Cu3Ox composition and 1 wt% CeO2 addition was studied. It is shown that the window for isothermal growth of single-crystalline sample grown from Sm123 seed is between 988 degrees C and 1003 degrees C. At lower temperatures of this window, Y123 crystals grow with higher growth rate but the quality of crystals is worse due to the formation of steps at the growth front. The optimum temperature 996 degrees C is proposed at which medium growth rate and better crystal quality are obtained. The growth rates in the a- and c-direction were measured and their dependence on undercooling is demonstrated. It is also shown that the growth rate under isothermal conditions is not constant and an epitaxially grown layer is formed during furnace cooling from the temperature of isothermal growth followed by the layer of misaligned Y123 crystals. The rest of not-solidified part of the sample solidifies by spheroidal growth. (C) 2012 Elsevier B.V. All rights reserved.