Spoctrophotometric study of the valence of rare earth ions in catalyst system Ln(napb)₃-Al(i-C₄H₉)₃-Al₂(C₂H₅)₃Cl₃ has shown that ions of Pr, Nd, Gd, and Er in the catalyst system are in a valence state of 3, and the difference in Choir oatalytio activity toward isoprene polymerization is not due to the difference in their valence state. The catalytio activity of the system Ln(napb)₃-Al(i-C₄H₉)₃-Al₂(C₂H₅)₃Cl₃ is related to the absorption intensity at 582 mμ. The stronger is the absorption, the higher is the catalytic activity. With increasing Al/Ln molar ratio and ageing time of the catalyst, the absorption intensity at 582 mμ and the catalytic activity are also inoreased.