The subsolidus phase relations in the Rb–Sr–U–O system were determined at 900^°C. Two new quaternary phases Rb₂Sr₂U4O₁₅ and Rb₈Sr₂U₆O₂₄in the Rb–Sr–U–O system were synthesized by heatin gthe respective oxides at 900^°C in air. A pseudo-ternary phase diagram of Rb₂O–SrO–UO₃ was drawn usin gthe new quaternary compounds, and various phase fields were established by X-ray powder diffraction analysis. X-ray powder diffraction (XRD) data of Rb₂Sr₂U₄O₁₅ were indexed by a monoclinic system with cell parameters,a=0.7875(4) nm,b=1.3199(4) nm,c=0.6667(5) nm and β=104.93(8)°, whereas XRD data of Rb₈Sr₂U₆O₂₄ were indexed by a cubic cell with a=0.8743(1) nm. The structure of Rb₈Sr₂U₆O₂₄ was derived from the powder data and structural parameters were refined by the Rietveld profile method.