GdCa4O(BO3)(3) (GdCOB) and YCa4O(BO3)(3) (YCOB) single crystals have been grown from the melt by the Czochralski method. Subsequent DTA analysis of the single crystals showed different behavior for both substances. During the first heating both crystals showed a single sharp melting peak at 1490degrees-C (GdCOB) or at 1504degreesC (YCOB), respectively. In subsequent heating/cooling runs only GdCOB shows one single peak, whereas YCOB shows five peaks. Moreover, phase separation of the melt can be observed by optical observation and by EDX measurements. This behavior can be explained by a miscibility gap in the YCOB melt. The YCOB crystal is formed from the stoichiometric melt by a monotectic reaction. (C) 2002 Elsevier Science Ltd. All rights reserved.