Three oxide melts of composition 10-20-70, 30-13-57, and 40-20-40 (wt%) in the ternary system CaO-Al2O3-SiO2 were equilibrated at 1700 K in platinum or platinum-rhodium boats, under oxygen partial pressures (PO2) ranging from 10(-13) to 1 atm, and fixed by various gaseous mixtures (H-2/ H2O, CO/CO2, and Ar/O-2). The melts were analyzed for platinum and rhodium content after quenching. The complex dependence of solubility of these elements with P-O2 was explained by their speciation, Chronopotentiometric measurements confirmed the passivation of the metallic boats. For increased P this speciation followed the series Pt, PtO, PtO2 or RhO for the solid phases covering the metallic boats and Pt-0, pt(2+), Pt4+, pt(6+) or Rh-0, Rh2+, Ph3+, Rh4+ in solution in the oxide melt. The predominance diagrams were determined for the three compositions in the ternary system.