Solid solutions of diphosphates of zinc and copper and of zinc and cobalt were synthesized from mixtures of pure diphosphates at temperatures up to 1000°C. Their X-ray diffractometry patterns varied continuously from one end member to the other. Solid solutions of orthophosphates of composition Zn 3−x Co x (PO 4 ) 2 , with x = 0.4-1.6, were formed at temperatures up to 950°C; all exhibited the structure of ␥-Zn 3 (PO 4 ) 2 . Solid solutions of orthophosphates of composition Zn 3−x Cu x (PO 4 ) 2 exhibited more-complex behavior. At 1000°C and copper contents of 20-80 mol%, a phase that is related to Cu 3 (PO 4 ) 2 , termed here the ''-phase,'' predominated. At 850°-950°C and in the region from 20 mol% to ∼33 mol% of copper, the solid solutions (the ''phase'') adopted the structure of graftonite. At 800°-900°C and 10-15 mol% of copper, the solid solutions exhibited a new structure (the ''␦-phase''), which we found to be related to the mineral sarcopside. At temperatures ≥950°C, the solutions that contained 5-15 mol% of copper (the ''phase'') had the structure of -Zn 3 (PO 4 ) 2 , whereas at 800°-850°C, solutions with 5 mol% of copper (the ''␥phase'') exhibited the structure of ␥-Zn 3 (PO 4 ) 2 . Attempts to synthesize Cu + ZnPO 4 and Cu + Cu 2+ Zn 3 (PO 4 ) 3 were unsuccessful.