The spinodal and, hence, the miscibility gap of relatively short chain polystyrene/polybutadiene (PS/PB) mixtures is shifted by 14 "C towards higher temperatures, if the hydrogen in PB is replaced by deuterium. The shift corresponds to a difference in the heat of mixing of about 0,2 kJ per mole of styrene units for a 1 : 1 PS/PB mixture. The effect has been observed by means of Gordon's Pulse Induced Critical Scattering method and the resulting spinodals are found to differ also with respect to the location of their maximum which indicates disparity in the entropy of mixing. For the prediction of the spinodal in a polystyrene/polybutadiene/perdeutereous polybutadiene (PS/PBH/PBD6) system the assumption of non-athermal mixing in PBH/PBD6 is needed. The value estimated for the PBH/PBD6 interaction parameter involves the second virial coefficient for solutions of PBD6 in PBH to be very close to zero. Expressions are presented for the second virial coefficient in quasiternary systems by scattering methods.Very recently we became aware of a theoretical treatment by Buckingham and Hentschel of the partial miscibility of liquid mixtures of hydrogenous and deutereous high polymerss3). These authors conclude that the heat of mixing must be expected to be positive, but so small that the chains must have a considerable length before partial miscibility of the upper critical type can occur. Assuming the gz3 value discussed above not to depend on the chain lengths we can calculate for a symmetrical case (mz = m3 = m) that chains longer than rn = 104 would not mix in all proportions. It is of interest to note that the experimental heats of mixing reported by

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