[en] Phase and crystal structure studies performed on the Pu-Pt, Pu-Rh, and Pu-Pt-Rh systems are described. The techniques employed included metallography, X-ray diffraction, and an electron microprobe. Results of these studies are correlated with previous work to obtain phase diagrams for these three systems. Previously unreported PuPtsub(3-kappa) and PuPtsub(3-kappa) polymorphs and a new PuPt₄ phase were identified. Eutectic points in the Pu-Pt and Pu-Rh systems were determined at 89.9 at%Pt and 84.5 at%Rh, respectively. No low melting eutectic alloys or ternary compounds were observed in the Pu-Pt-Rh system. Crystal structures determined by X-ray diffraction measurements were as follows: PuPt₅ (hexagonal, P 6/mmm, CaCu₅-type); PuPt₄ (orthorhombic, Cmmm); and PuPtsub(3-kappa) (BC tetragonal, DOsub(e), PNi₃-type). (Auth.)