We have observed large hydrogen-deuterium isotope effects on the microphase separation that occurs in unstable crystalline binary mixtures of n-alkanes rapidly quenched from the malt. These effects were studied in C-30/C-36 mixtures at room temperature, mainly by infrared spectroscopy and electron diffraction. The isotope effects are manifest in a number of different ways : (i) immediately after the quench, the D/H (C-30(D)/C-36(H)) mixture is significantly more demixed than the H/D (C-30(H)/C-36(D)) mixture; (ii) the spontaneous demixing, which begins immediately after the quench to room temperature, is faster for D/H by a factor of about 3; (iii) the C-36(H) domains in the D/H mixture grow at a notably faster rate than those of C-30(D) and at a faster rate than the domains of either component in the H/D mixture. These effects can be largely accounted for in terms of molar volume differences between the hydrogenated and deuterated n-alkanes that constitute the mixtures. Similar isotope effects are found to occur in other binary n-alkane mixtures.