Crystal structure, superconductivity, and magnetic order of the Pr spins in polycrystalline (Pr0.55Nd0.45)Ba2Cu3O7 were Studied by x ray and ac susceptibility measurements. The crystal structure was determined by applying the Rietveld refinement method to the high resolution x-ray diffraction pattern. The Pr and Nd atoms are randomly mixed to form (Pr0.55Md0.45) layers rather than two separated PrBa2Cu3O7 and NdBa2Cu3O7 phases. Above 50 K the temperature dependence of the in-phase component of the ac susceptibility chi'(T) exhibits Curie-Weiss behavior. Below 50 K diamagnetic responses become evident, indicating the onset of superconductivity. A dc field of strength 2 kOe smears out 80% of the diamagnetic signals, showing the superconducting pairings are being destroyed by the applied field. Further reducing the temperature, chi'(T) shows an upturn at T approximate to 15 K below which chi'(T) increases rapidly with decreasing temperature, presumably due to the Pr spin ordering. Both the onset of superconductivity and the spin ordering of the Pr ions are also evident in the temperature dependence of the out-of-phase component of the ac susceptibility chi''(T). (C) 1997 American Institute of Physics.