Phase relations of the Pt-Pd-Sb system were investigated at 1000 °C, with an emphasis on the extent of Pt-Pd substitutions in Pt and Pd antimonide minerals and their stable assemblages. The sealed capsule technique was used, and the reaction products were examined using reflected light microscope, X-ray powder diffractometer, and electron microprobe. Pt-Pd alloy minerals, stumpflite (PtSb), geversite (PtSb₂), mertieite II (Pd₈Sb₃), and stibiopalladinite (Pd₃₁Sb₁₂) occur as solid phases. In addition, solid phases with compositions of Pd₂₀Sb₇ and Pd₅Sb₂ are present. The extents of the Pd substitution for Pt in stumpflite and geversite were measured to be 18 and 7.2 atom (at.) %, respectively. Mertieite II (Pd₈Sb₃) and stibiopalladinite (Pd₃₁Sb₁₂,) show almost identical extent of the Pt-Pd substitution (22 at.%). The Pt substitutions for Pd in synthetic phases of Pd₂₀Sb₇ and Pd₅Sb₂ range up to 27 and 19.6 at.%, respectively. Pt and Pd form a complete solid solution and their solid solutions with Sb range from 7.5 at.% along the Pt-Sb binary join to 16.5 at.% along the Pd-Sb binary join.