The complexes of rare earth elements with 1,2-diphenyl-4-butyl-3,5-Pyrazolidinedione (PBH, phenylbutazone) were synthesized and characterized by elemental analysis, molar conductance, IR, UV-Vis, EPR and magnetic moment measurements. Based on these studies the complexes were formulated as [Ln(PB)3(H2O)2]?nH2O, where Ln=Eu(III), Gd(III), Tb(III), Dy(III) and Er(III). From IR spectra, it was found that PBH acted as a bidentate mono-ionic ligand coordinating through two carbonyl oxygen of the pyrazolidinedione ring. The thermal analysis of all the complexes was carried out at a heating rate of 10 oC/min. The kinetic aspects of the complexes were evaluated. The nega-tive entropy value of the complexes indicated a more ordered state for the activated complexes. The photoluminescence property of Tb(III) complex was investigated. It showed all the characteristic emission peaks of Tb3+ with a life time of 0.98914 ms. The complexes of rare earth elements with 1,2-diphenyl-4-butyl-3,5-Pyrazolidinedione (PBH, phenylbutazone) were synthesized and characterized by elemental analysis, molar conductance, IR, UV-Vis, EPR and magnetic moment measurements. Based on these studies of complexes were formulated as [Ln (PB) 3 (H2O) 2] nH2O where Ln = Eu (III), Gd (III), Tb From IR spectra, it was found that PBH acted as a bidentate mono-ionic ligand coordinating through two carbonyl oxygen of the pyrazolidinedione ring. The thermal analysis of all the complexes was carried out at a heating rate of 10 oC / min. The kinetic aspects of the complexes were evaluated. The nega-tive entropy value of the complexes indicates a more ordered state for the activated complexes. The photoluminescence property of Tb (III) complex was investigated. It showed all the characteristic emission peaks of Tb3 + with a life time of 0.98914 ms.