Disclosed herein are solution polymerization processes for producing a high-cis polydiene. The processes include polymerizing at least one conjugated diene monomer in an organic solvent in the presence of at least one thiol compound and a lanthanide-based catalyst composition to produce a polydiene having a cis-1,4-linkage content of 90-99%. The at least one thiol compound is represented by the general formula R—S—H, where R is a hydrocarbyl group or a substituted hydrocarbyl group. The lanthanide-based catalyst composition comprises (a) a lanthanide compound, (b) an alkylating agent, and (c) a halogen source, where (c) may optionally be provided by (a), (b), or both (a) and (b). The molar ratio of the at least one thiol compound to the lanthanide compound used in the polymerization process ranges from 0.01:1 to 100:1. Improved solution polymerization processes regarding the same are also disclosed herein.