Abstract: A block copolymer comprising a polymer block of an aromatic vinyl compound, a polymer block of butadiene and a polymer block of isoprene, having a weight average molecular weight of 10,000-500,000, containing 10-50 wt. % of a bound aromatic vinyl compound, 10-60 wt. % of bound butadiene, 10-60 wt. % of bound isoprene, at most 15 wt. % of 1,2-vinyl bonds in a butadiene segment and at most 10 wt. % of 3,4-vinyl bonds in an isoprene segment, and having a storage modulus (G') of 10.sup.6 -3.times.10.sup.8 Pa in a viscoelasticity measurement in a temperature range of from 0.degree. C. to 50.degree. C. and only one peak on loss tangent (tan .delta.) attributable to the butadiene polymer block and the isoprene polymer block in a temperature range of from -80.degree. C. to -50.degree. C. scarcely undergoes a change of melt viscosity with time. A pressure sensitive adhesive composition comprising the block copolymer and a tackifier is also provided.

Abstract: A block copolymer composition comprising 20-90 wt. % of at least one block copolymer selected from the group consisting of block copolymers represented by the general formula (S--B--I).sub.n --X and block copolymers represented by the general formula (S--I--B).sub.n --X, and 80-10 wt. % of at least one block copolymer selected from the group consisting of block copolymers represented by the general formula S--B--I and block copolymers represented by the general formula S--I--B, and a pressure sensitive adhesive composition comprising, as a base polymer, the block copolymer composition are provided.