Relationship between adhesion properties and phase structures of styrene triblock and diblock copolymer blends was investigated in detail. For this purpose, polystyrene-block-polyisoprene-block-polystyrene triblock and polystyrene-block-polyisoprene diblock copolymers were used and the diblock content was varied from 0 to 100 wt All blends formed the sea-island structure in which spherical polystyrene domains were dispersed in polyisoprene matrix and mean domain size was 20 nm. The domain size was slightly affected by the diblock content. The fracture stress and strain measured by a tensile test decreased and the molecular mobility measured by a H-1 pulse nuclear magnetic resonance analysis increased with an increase of diblock content. The tack as adhesion property increased with an increase of diblock content below 70 wt %), then decreased over 70 wt To. The cohesive strength decreased and the interfacial adhesion increased with an increase of diblock content. The tack increases by the development of cohesive strength and interfacial adhesion. Therefore, the tack showed the maximum at the optimum contribution balance between cohesive strength and interfacial adhesion. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 1766-1773, 2010