The high-temperature crystallizing phases of fluxes (molten solutions) based on Bi2O3:MoO3 = 1:3 and diluted by Na2CO3 and B2O3 (under varying of Na2CO3:B2O3), and by CuO and Mn2O3 (0 ≤ Mn2O3:CuO ≤ 5) are studied. The conditions of the stable crystallization of Mn2+ 1−xCuxMoO4, Mn2 3+O3, Mn2+Mn2 3+O4, Mn2+ 1−xCuxMn3+BO4 and Mn2+ 2−xCuxMn3+BO5 have been found. The influence of MoO3 and Na2CO3 to the crystallization processes of Mn2+- and Mn3+-containing Mn2+ 1−xCuxMoO4 and Mn2+Mn2 3+O4 and Mn2 3+O3 are clarified. The occurrence of the chemical bonds of these types at the crystallization of Mn-heterovalent Mn2+ 1−xCuxMn3+BO4 and Mn2+ 2−xCuxMn3+BO5 oxyborates is studied. Single crystal growth techniques of these oxyborates are suggested. The results of the structural and magnetic characterization of some discussed compounds are presented.