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The effects of cerium and ruthenium additions on the mechanical properties of Pt-15Pd-3.5Rh alloy (wt.%) at ambient and high temperature are examined. The mechanical properties of the alloy were improved by adding cerium (≤0.1 wt.%) or rathenium (≤0.5 wt.%) solute, with the cerium additions giving a better strengthening effect. Higher concentrations of cerium and ruthenium did not visibly increase the strength properties of the Pt-15Pd-3.5Rh alloy and even reduced some of the mechanical properties at high temperature. Increasing the palladium content in the Pt-Pd-Rh alloy could enhance the alloy strengths at room temperature but damaged the mechanical properties at high temperatures. Observations of the morphologies of fracture sections of alloy samples after creep-rupture tests at high temperature showed ductile fracture for alloys with lower contents of palladium, cerium or ruthenium and brittle fracture for alloys with higher contents of palladium, cerium or ruthenium. The different strengthening mechanisms of palladium, cerium and ruthenium additions to Pt-15Pd-3.5Rh alloy are discussed.