This paper involves the examination of refractive index increments and UV absorption spectra of styrene-isoprene diblock copolymers in CHCl3. It is found that the block copoly-mers

just as with the blends of corresponding homopolymers

have good An/Ac additivity

indicating that dn/dc measurement can be used to determine the composition of block copolymers. On the other hand

the results of UV absorption spectroscopy show that block copolymer samples exhibit marked UV hyperchromism

except one sample with low polystyrene content. The-refore

UV

and UV-RI dual detector GPC will yield unreliable composition data of the block copolymers. The hyeperchromic phenomena of these block copolymers are closely related to their peculiar UV absorption spectra observed.