Abstract: In a process for separating a conjugated diene from a hydrocarbon mixture containing the conjugated diene by extractive distillation using dimethyl formamide as an extractive solvent, a method for inhibiting polymerization of the conjugated diene in the dimethyl formamide at a high temperature which comprises using furfural and furfural polycondensates as polymerization inhibitors, and controlling the amounts of the polymerization inhibitors such that about 0.01 to about 2% by weight, based on the solvent, of furfural and about 0.5 to about 10% by weight, based on the solvent, of the furfural polycondensates are present in a total amount of about 1 to about 12% by weight, based on the solvent, throughout the entire separating process.