A series of amorphous binary mixtures of normal and perdeuterated 1,4-polybutadienes has been examined by small-angle neutron scattering (SANS). Contrary to the generally held assumption that isotopic polymer mixtures form ideal solutions, we conclusively demonstrate that these mixtures are characterized by an upper critical solution temperature (UCST). This isotope effect derives from a small, measurable difference in segment volume between normal and perdeuterated species. Above the UCST, the SANS results are quantitatively predicted by the mean-field theory of de Gennes for homogeneous binary polymer mixtures. Increasing the degree of polymerization raises the critical temperature, resulting in phase separation. Owing to the combined effects of the close proximity to the consolute point and a small segment–segment interaction parameter (x ≃ 10–3), the phase-separated mixtures exhibit extensive interfacial mixing; Porod analysis of the SANS results reveals an average interfacial thickness of (D) = 250 A. Overall, these findings demonstrate that normal and perdeuterated amorphous polymers represent a new class of materials with which to study polymer–polymer phase behavior.