Abstract: The thermodynamics of equilibrium polymerization of alpha-mclhylslyrene had been investigated. It was observed that the equilibrium monomer concentration[M]_e is dependent not only on the temperature but also on the initiator concentration [BuLi]_o.This phenomenon could be easily elucidated by the mechanism of polymerization proposed, and the mathematic relationship batween [M]_e and [BuLi]_o was obtained.Furthermore, the equilibrium constants of anionic equilibrium polymerization K₁, K₂, the free-energies ΔG₁, ΔG₂, heats of polymerization ΔH₁, ΔH₂,and entropies of polymerization ΔS₁, ΔS₂, of monomenc and associated reactive species were reported.