Abstract Compositional inhomogeneity induced by the Soret effect was studied in two Zr-based bulk metallic glasses (BMGs): Zr 50 Cu 50 and Zr 50 Cu 40 Al 10 (at%), and one Cu-based BMG: Cu 60 Zr 30 Ti 10 (at%), all of which were prepared by rapid solidification. The concentration of Cu increases from the surface to the interior, while the concentrations of Zr, Ti and Al decrease. The magnitude of the Soret effect is found to be highly dependent on the sample size and interactions between the diffusing atoms in bulk metallic glasses. For the Zr 50 Cu 50 alloy, a large sample size favors the Soret effect, because of the longer diffusion time it affords compared to a small sample. Further, the additions of Al and Ti in the Zr–Cu BMGs reduce the magnitude of the Soret effect by the formation of short-range order and/or inter-atomic clusters.