Recently, we demonstrated that amorphous mixtures of normal (protonated) and perdeuterated 1,4-polybutadienes are characterized by an upper critical solution temperature (UCST), contradicting the widely held assumption that such isotopic mixtures form ideal solutions. This effect was shown to derive from a small difference in segment volume between the perdeuterated and normal species, as predicted by Buckingham and Hentschel. In the symmetric case (ND = NH= N), the predicted critical degree of polymerization for amorphous isotopic polymer mixtures is where kBis the Boltzmann constant, T is the UCST, and βTis the isothermal compressibility. V represents the average segment volume for the mixture, and ΔV is the difference in volumes between the normal and perdeuterated (undiluted) segments. On the basis of the symmetric version of the Flory-Huggins approximation to the mixing free energy, the critical degree of polymerization is given by where x is commonly referred to as the “segment-segment interaction parameter”. We therefore expect all mixtures of normal and perdeuterated polymers to exhibit a small positive interaction parameter, x ~ T-1, since such isotopic substitution produces changes in molecular volume in essentially all organic compounds. In this communication we report our initial findings concerning the phase behavior of amorphous mixtures of perdeuterated and normal atactic polystyrenes. These results are consistent with our expectation of a universal isotope effect in amorphous polymers.