[Pt(NH3)(4)](2+) and [Rh(NH3)(5)Cl](2+) were ion exchanged into NaX or NaY either individually or simultaneously in Pt/Rh molar ratios 1:1 and 1:5. Carbonylation of the Pt ammine in NaX at 75-100 degreesC leads to the orange [Pt-3(CO)(6)](2)(2-) carbonyl and that of Ph ammine to the greenish [Rh-6(CO)(15)](2-). Under the same reaction conditions, the mixture of both ammines (1:5) reacts to an almost white or pale beige bimetallic carbonyl, assigned to [PtRh5(CO)(15)](-). This is also formed from the (1:1) mixture with the excess of Pt ammine carbonylated to [Pt-3(CO)(6)](2)(2-). The same bimetallic monoanionic carbonyl is formed in less basic NaY, although Ph ammine alone reacts in this matrix very slowly only to the neutral Rh-6(CO)(16) complex. Oxidation of [PtRh5(CO)(15)](-) at 100 degreesC extracts some Rh(CO)(2)(+) from the bimetallic carbonyl, but fast recarbonylation to the original species occurs.