We have investigated the thermodynamic interactions as functions of component volume fraction phi and temperature T in six binary polymer blend systems. The components in all cases were model polyolefins, made by saturating the double bonds of nearly monodisperse polydienes. Small-angle neutron scattering measurements for single-phase melts, analyzed with the incompressible random phase approximation, were expressed in terms of the Flory-Huggins interaction parameter chi for 27 degrees C less than or equal to T less than or equal to 167 degrees C and 0.1 less than or equal to phi greater than or equal to 0.9. In most systems we found a characteristic upturn in chi at the composition extremes that diminished with increasing temperature. We also found that the component chain dimensions were unaffected by the strength of the interactions. Several theoretical attempts to explain the dependence of chi on component volume fraction were examined. Most were qualitatively inconsistent with the results, and none were fully satisfactory.