Diffusion and sorption of cyclohexane vapor in styrenebutadiene copolymers were used to study the morphology of an SBS triblock copolymer (29.3 wt% styrene) cast from different solvents. The weight gains or losses of this SBS and several SBR (5, 23, 45, and 85 wt% S content) samples were continuously monitored at 40°C and at penetrant pressures of 30, 60, 90, and 120 Torr. Significant differences exist in the equilibrium sorption among the different samples. Possible reasons for this observation are explored. The one corroborated by dynamic mechanical measurements attributes the differences to varying interphase volume fractions. The interphase region separating the PB and PS phases in SBS was assumed to be a continuous profile of equivalent SBR with different S contents. Experimental results, coupled with simple calculations, indicated that the overall interphase volume fractions in the SBS are 0.28 for the MEK/THF-cast (1:9 volume ratio of MEK and THF) sample, 0.15 for the toluene-cast sample, and 0.06 for the cyclohexane-cast sample. Important observations consistent with these results are: (1) the existence of a relative minimum (at around 20–30 wt% S content) in the plot of the equilibrium sorption of cyclohexane vs S content in rubbery SBR; and (2) a relative maximum in the plot of the mutual diffusion coefficient of cyclohexane in SBR vs the concentration of cyclohexane in SBR.