High molecular weight, block copolymers of styrene, butadiene and isoprene were prepared in tetrahydrofuran solution by anionic polymerization techniques employing sodium-α-methylstyrene initiation. Selected samples were characterized by dilute solution techniques and found to be genuine block copolymers with narrow molecular weight distributions and little composition heterogeneities. The multiphase nature and the mechanical response of the polymers were investigated by torsion pendulum analyses. The two phase nature of the styrene-butadiene block copolymers was evident from the two glass transitions characteristic of the individual segment types, polystyrene and polybutadiene. The purity of the phase separations was indicated by the invariance of the butadiene T g with copolymer composition and the equality of the mechanical response of two copolymers with identical compositions but inverted monomer sequences, i.e. ABA versus BAB. Variations in the magnitude of the butadiene loss peak and the step character of the log modulus/temperature curves were also determined as functions of the composition. Some effects of macrophase size were observed by comparing the mechanical behaviour of a physical blend of homopolymers with a block copolymer of the same composition. The temperature variation of the modulus was greatly affected above the butadiene T g. The compatibility of similar soft segments, polybutadiene and polyisoprene was also investigated. Block terpolymers of styrene, butadiene and isoprene were compared to a physical mixture of styrene-butadiene and styrene-isoprene block copolymers. Although the blending of the two soft diene segments in the physical mixture was substantial, it was not as complete as that obtained through terpolymerization where some indication of the effect of segment position on the blending was also noted.