Abstract: The positron annihilation technique was used to study the properties of styrene-isoprene-styrene block copolymer in the temperature of - 80 to 100℃. In the temperature spectrum of the longest lifetime τ₃, in addition to two transition temperatures which correspond to the glass transition temperatures of polyisoprene and polystyrene, an irregularity was observed.This transition temperature is interpreted as the glass transtion temperature of the interphase in block copolymers.