Syntheses along the Mg3Al2(SiO4)3-Na3Al2(PO4)3 join were carried out in the 15–17 GPa range at temperatures between 1200 and 1600°C. An Na3Al2(PO4)3 compound of garnet-like structure, with a cubic-cell parameter of 11.579(2) Å, was synthesized and characterized. Intermediate compositions are also found to crystallize in the garnet structure. Between pyrope and Na3Al2(PO4)3, the cubic unit-cell volume increases by 3.1%. In addition, nominal compositions, Mg2.5Na0.5Al2Si2.5P0.5O12 and Mg2Na1Al2Si2P1O12, were run at 3 GPa and 875 °C. They yielded garnet, among other phases, but with no significant phosphorus incorporation as indicated by a pyrope-like unit-cell volume. Phosphorus content in pyrope from the UHP Dora-Maira quartzites appears as a potential pressure indicator where UHP garnet coexists with phosphate minerals. The P-content in upper mantle garnets can be used as a probe of phosphorus activity in the host rock.