Polymerization of isoprene was investigated by using a novel ternary catalyst system composed of neodymium(III) isopropoxide (Nd(Oi-Pr)(3)), dimethylphenylammonium tetrakis(pentafluorophenyl)borate ([HNMe2Ph](+)[B(C6F5)(4)](-); borate), and triisobutyl-aluminum (i-Bu3Al). The mole ratios of borate and aluminum compounds to Nd catalyst significantly affected the polymerization behavior. Both yield and cis-1,4 content of polyisoprene decreased in the case of [borate]/[Nd] < 1.0, while at [borate]/[Nd] > 1.0 the formation of multiple active species resulted in the polymer showing bimodal peaks in GPC. When the [Al]/[Nd] ratio was lower than 30, the polymer yield sharply decreased, whereas the cis-1,4 content became relatively low with use of a large excess of Al ([Al]/[Nd] > 50). Thus, the optimal catalyst composition was [Nd]/[borate]/[Al] = 1/1/30, which gave in >97% yield polyisoprene with high molecular weight (M-n similar to 2x 10(5)) and relatively narrow molecular weight distribution (M-w/M-n similar to 2.0) and mainly cis-1,4 structure (similar to 90%).