Ultrahigh cis-polybutadiene is prepared in 1,3-butadiene polymerization by the Nd catalyst of Nd(neodecanoate)(3) (ND)/AlEt2Cl/Al(Bu-i)(3) in the rubber industry. However, the catalytic activity remains low, and the reason is unclear. According to our matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopic study, ND is a mixture of various oligomeric and hydrated compounds. The hydrated oligomeric structure would mainly lower the catalytic activity. To design a better Nd catalyst, Nd(neodecanoate)(3)(.)(neodecanoic acid) (NDH) was designed to satisfy the requisite of anhydrous and monomeric structure. NDH was prepared through ligand-exhange method between neodymium acetate and neodecanoic acid in an organic medium. NDH was identified and characterized with infrared JR), MALDI-TOF mass, and synchrotron X-ray absorption spectroscopies (XAS) and with computer simulations. Weakly bound neodecanoic acid with the carbonyl peak at ca. 1670 cm(-1) and carboxylate anion coordinated to Nd at ca. 1600 cm-1 were observed in the IR study. The molecular ion peak (m/z = 838.7) in MALDI mass and the pronounced absorption edge at 6216 eV in the Nd L-III XANTES (X-ray absorption near edge structure) spectra support its monomeric structure. NDH is of monomeric structure, satisfying the 8-coordination number in the optimized structure by molecular mechanics. A significant improvement in the catalytic activity for 1,3-butadiene polymerization is achieved through NDH. The Nd catalyst shows a high activity (2.5 x 10(6) g/(Nd mol h)) and produces polybutadiene with 98% cis content.