The temperature dependence of specific volume was measured for miscible polybutadiene/polyisoprene blends by dilatometry. The negative excess volume of mixing below the lower critical solution temperature was studied in terms of the Flory equation-of-state. In order to achieve a best-fit of the calculated excess volume to the experimental excess volume, Flory’s interaction parameter X12 was estimated to be negative for the blend. Thus, a stereo structural effect for mixing would be predicted, since only van der Waals dispersion forces are expected between blend components. Considering that one way to observe the stereo-structural effect is to study conformational changes during mixing, solid-state proton NMR measurements were conducted. Evidence of a conformational change was obtained for the blend. The occurrence of a structural effect was also suggested.