The sub-solidus phase relations in SrO–UO₃–MoO₃ system were determined at 1023 K in air. Various compounds in SrO–UO₃, UO₃–MoO₃, SrO–MoO₃ and Sr–Mo–U–O systems were synthesized by solid state method, reacting SrCO₃(s), UO₃(s) and MoO₃(s) in desired molar proportions at 1023 K. A pseudo-ternary phase diagram of SrO–UO₃–MoO₃ system was drawn on the basis of phase mixtures obtained in the present work. A single quaternary compound, SrUMo₄O₁₆(s) was identified. The compound melts at 1043 K and decomposes above 1173 K to give SrMoO₄(s) and U₃O₈(s). SrUMo₄O₁₆ showed positive thermal expansion under vacuum from ambient to 873 K with an average volume expansion coefficient α_v = 9.73X10^-6 K^-1. The molar heat capacity of SrUMo₄O₁₆(s) was measured using differential scanning calorimeter. Using the thermodynamic data, oxygen, MoO3 and strontium potential diagrams of SrO–UO₃–MoO₃ system have been computed at 1023 K.