We use cross-sectional scanning tunneling microscopy (STM) to reconstruct the monolayer-by-monolayer composition profile across a representative subset of MBE-grown InAs/InAsSb superlattice layers and find that antimony segregation frustrates the intended compositional discontinuities across both antimonide-on-arsenide and arsenide-on-antimonide heterojunctions. Graded, rather than abrupt, interfaces are formed in either case. We likewise find that the incorporated antimony per superlattice period varies measurably from beginning to end of the multilayer stack. Although the intended antimony discontinuities predict significant discrepancies with respect to the experimentally observed high-resolution x-ray diffraction spectrum, dynamical simulations based on the STM-derived profiles provide an excellent quantitative match to all important aspects of the x-ray data. (C) 2015 Elsevier B.V. All rights reserved.