Chemical reactions play an important role in many crystal growth processes. Often, the knowledge of phase relations is sufficient in order to control the crystallization. In the case of the crystal growth of YBa₂Cu₃O_x in nonstoichiometric BaO/CuO melts chemical reactions can cause dramatic changes of the phase relations. This article deals with the redox reactions of the CuO-component and the high reactivity of strong basic melts with surrounding media. Knowing these reactions one can utilize them to support the crystal growth and/or to reduce the negative effects.