Small-angle neutron scattering (SANS) experiments were conducted on a 25 vol % blend of a poly(ethylene-block-head-to-head propylene) copolymer (PE-PP) in polyisobutylene (PIB). PE/PIB and PE/PP chains are incompatible (the Flory-Huggins interaction parameter chi>0), while PIB and PP are compatible (chi<0) at low temperatures and incompatible (χ>0) at high temperatures. These interactions lead to an order-disorder transition at 150+/-5degreesC and macrophase separation at 251+/-5degreesC. The nature of the concentration fluctuations in the homogeneous state (155degreesC