Stress, strain, and optical properties of three elastomeric styrene butadiene block copolymers containing 31, 40 and 49 wt per cent styrene were studied as a function of temperature. Mechanical and optical properties indicate that these materials are two phase systems in which the polybutadiene chains form an elastomeric phase and the polystyrene a glassy phase with the latter providing physical crosslinks. Birefringence measurements indicate that decreases in modulus and strength of these materials are associated with decrease in concentration of elastically effective network chains. Independence of stress-optical coefficient of temperature suggests that the decrease in concentration of elastically effective chains is not due to onset of rubberlike behavior or flow within the polystyrene regions themselves, at least for temperatures below about 70° C. Rather, the decrease seems to be associated with increased mobility of the polybutadiene chains at higher temperatures, which also leads to an increase in the rate of stress relaxation. Birefringence measured during extension and retraction showed that stress strain hysteresis is due to restricted mobility of polybutadiene chain segments rather than to permanent viscous flow or to change in the effective network structure of the block copolymers. The ultimate properties of these rubbers were well correlated with the effective network structure in undeformed specimens.