Phase formation in the YbAl3(BO3)(4)-K2Mo3O10-B2O3-Yb2O3 complex fluxed melts within the field of YbAl3(BO3)(4) (YbAB) stability has been investigated in the temperature interval from 1150 to 1000 degrees C. The region of primary YbAB crystallization was revealed for 15 wt% YbAB cross-section in this pseudo-quaternary system. A correlation between YbAB solubility and B2O3, K2Mo3O10 and Yb2O3 contents in the solvent has been found. Precise concentration-temperature diagrams were plotted by YbAB saturation points for two technologically significant flux Compositions: (I) 55 mol% K2Mo3O10-45 mol% B2O3 and (II) 55 mol% K2Mo3O10-40 mol% B2O3-5 mol% Yb2O3. The data obtained were analyzed in comparison with those for YAl3(BO3)(4) (YAB) as another end member of the YbAB-YAB solid solutions. (C) 2008 EIsevier B.V. All rights reserved.