The effect of saturation on thermodynamic interactions in polystyrene-polyisoprene (SI) diblock copolymers is examined by small-angle neutron scattering (SANS) and birefringence. Experiments were conducted on an SI sample in which both blocks had molecular weights of 7.5 x 10(3) and its saturated product-a poly(vinylcyclohexane)-poly(methylbutylene) block copolymer (HSHI). The SANS data show a reversal in miscibility in the vicinity of 96-degrees-C. Above this temperature, the Flory-Huggins interaction parameter, chi, is higher in the HSHI polymer than in the SI polymer, while the reverse is true at temperatures below 96-degrees-C. The high-temperature result is consistent with published data of Gehlsen and Bates, while the low-temperature result is confirmed by birefringence measurements presented here.