In narrow-gap semiconductors, non-radiative Auger recombination losses (via conduction, heavy-hole, spin split-off hole, heavy-hole processes) are sensitive to how the band gap energy, E-0, compares to the spin-orbit splitting, Delta(0). However, narrow-gap band structures have often not been very well characterised. We report on the mid-infrared photo-modulated reflectance of E-0 and E-0 + Delta(0) transitions in high-quality, InAs-rich InAsSb and GaInAsPSb samples as functions of both temperature (T = 10-300 K) and antimony content <= 22.5%, for wavelengths up to similar to 4.75 mu m. The measured T-dependence of Eo is generally consistent with that accepted in the literature and we confirm that Delta(0) is T-independent. As a function of Sb fraction, E-0 is consistent with the positive bowing parameter of +670 meV quoted in the literature. However, Delta(0) does not exhibit the Currently accepted positive bowing parameter of + 1170 meV: rather, a best fit to our data tentatively Suggests a negative bowing of similar to-165 meV Due to the importance of An in predicting and interpreting the performance of InAsSb-based devices, this result is expected to have a significant impact. (C) 2008 Elsevier B.V. All rights reserved.