The applicability of the dual constant composition (DCC) method, using two ion-specific electrodes, is investigated for studies of the dissolution rates of mixed phases of octacalcium phosphate and tricalcium phosphate. The DCC dissolution of these crystals is compared with the results of parallel constant composition (CC) investigations using a single-ion probe. The DCC dissolution rates were also independent of which electrodes (pH or pCa) controlled the titrant additions. This provides experimental confirmation that, in the concomitant dissolution of hypothetical BA and BC salts (e.g., octacalcium phosphate and tricalcium phosphate), the rate data for each salt can be calculated from the titrant addition curves in the same undersaturated solutions.,