Single crystalline GdCa4O(BO3)(3) and YCa4O(BO3)(3) show one DTA melting peak at t(f)(Gd) = 1490 degreesC or t(f)(Y) = 1504 degreesC, respectively. Only pure GdCa4O(BO3)(3) and mixed crystals Gd1-xYxCa4O(BO3)(3) with x less than or equal to 0.46 showed one single peak also during cooling or during subsequent heating runs. The melting points of such intermediate compositions can be described well assuming ideal behavior of the solid as well as of the liquid phase resulting in a conventional quasibinary phase diagram with complete solubility and a narrow two-phase region solid/liquid. Compositions with x greater than or equal to 0.5307 show a miscibility gap in the liquid phase. For solidified pure YCa4O(BO3)(3) (x = 1), the content of the crucible was found to consist of the congruently melting compounds Y2Ca3(BO3)(4), Ca-3(BO3)(2), Y2O3 and of the original YCa4O(BO3)(3). The first three compounds built up a partial system of the concentration triangle B2O3, CaO, and Y2O3.