Abstract: Thermoplastic elastomers of conjugated diolefins having 4 to 6 carbon atoms and monovinyl aromatic hydrocarbons are prepared by polymerizing 8 to 65 parts by weight of a monomer mixture A.sub.1 comprising a conjugated diolefin having 4 to 6 carbon atoms and a monovinyl aromatic hydrocarbon in a weight ratio of 85-10:15-90 in a hydrocarbon solvent and an inert atmosphere in the presence of 0.2 to 20 mmol., per 100 g. of the total amount of the mixture A.sub.1 and the mixture A.sub.2 described below, of a monolithium hydrocarbon calculated as active lithium to form an active copolymer, and after substantial completion of the polymerization, copolymerizing said active copolymer with 92 to 35 parts by weight of a monomer mixture A.sub.2 comprising the same conjugated diolefin having 4 to 6 carbon atoms and monovinyl aromatic hydrocarbon as said mixture A.sub.1 in a weight ratio of 95-20:5-80, such that the total amount of said mixtures A.sub.1 and A.sub.