We have examined the electrochemical reactions taking place at Ba2YCu3O7 and CuO in several aqueous media under anodic and open-circuit conditions. Understanding of the reactions of these materials in water containing environments may be important in their fabrication into devices. Ba2YCu3O7 is dissolved by aqueous acids at mass transport limited rates up to 0.1M in H+ concentration in 1M NaCl with no selectivity for metal ions, i.e., they are found in stoichiometric ratios. This conclusion was determined by analysis following controlled exposure of rotating disk electrodes of the high-temperature superconductor. When these disks are anodized at constant current in NaCl or NaClO4 media, essentially only Ba dissolves. In pH 6.9 phosphate buffer or 1M NaOH, lesser fractions of Ba are found in solution. Anodization in saturated CaCl2 produces chlorine and nonselective dissolution of the substrate simultaneously. Chemistry for these various situations is proposed.