A segmented ethylene terephthalate (ET)-caprolactone (CL) copolymer was characterized by light scattering in chloroform tetrahydrofuran and butanone. The flexibility of the copolymer chain is comparable with that of typical flexible chains, such as polystyrene. In the process of applying the Bushuk-Benoit light scattering theory to the segmented PET-PCL copolymer, we encountered not only the problem of finding three solvents with different refractive index but also the problem of determining the specific refractive index increments for the PET and PCL segments in the copolymer, i.e., nu(PET) and nu(PCL). In principle, the approximate values of nu(PET) and nu(PCL) can be obtained from the PET and PCL homopolymers, respectively. In reality, it involves many practical problems, e.g., to find three solvents not only for copolymer but also for the PET and PCL homopolymers. In this study, a different method was used to find both nu(PET) and nu(PCL), wherein the nu values of at least two segmented PET-PCL copolymers with different PET compositions were used. With nu(PET), nu(PCL), and nu, we characterized the absolute molecular weight. Further, we show that the composition of an unknown segmented PET-PCL copolymer can be estimated from nu(PET), nu(PCL), and nu.