The present invention is directed to a process for the preparation of a gel-free hydrocarbon solution containing a lanthanide rare earth series compound by dissolving or synthesizing a lanthanide compound in an inert hydrocarbon solvent with a Lewis acid in an amount sufficient to prevent formation of highly viscous solutions. The present invention is also directed to the use of the gel-free Ln solutions formed by the recited process as co-catalysts for the polymerization of dienes. The Lewis acid is a group 2, 12, 13, 14 or 15 halide or organohalide, or a transition metal halide, excluding aluminum trialkyls.