High-temperature order–disorder transformations in R2T17 and R2T17-M-C intermetallics with R = Pr, Sm, Dy, Tb; T = Co, Fe; and M = Zr, Nb were studied utilizing time-resolved synchrotron x-ray diffraction at the Advanced Photon Source (APS) at the U.S. Department of Energy’s Argonne National Laboratory (Argonne, IL). High-energy synchrotron radiation provides intense, highly penetrating x-rays, which are ideal for in situ studies of phase transformations. Alloying additions are used to stabilize formation of metastable phases; their influence on order recovery was investigated. The experimental setup utilized Debye–Scherrer geometry; specimens were heated at a rate of 10 K/min. Full-profile diffraction patterns collected every 10 s were refined in sequence using the Rietveld method to track changes of lattice parameters and phase assemblages during heating. Sharp changes observed in the evolution of temperature-dependent lattice parameters suggested formation of ordered structure via nucleation and growth. Both 2-17 polymorphs co-existed in light and heavy rare-earth systems at high temperatures. The presence of alloying additions in the solid solution greatly influenced long-range order formation.