The effects of structural and chemical disorder on electronic and optical properties of GaAlAsSb quaternary alloy are studied on the basis of a modified virtual crystal approximation calculated within a simple tight-binding sp(3)s* theory, which incorporates compositional disorder as an effective potential. Using a minimal set of fitting parameters, we show that such an approach provides analytical results for calculating energy gaps and bowing parameters. We show that the calculated bowing parameter agrees reasonably well with experimental data and other recent calculations. The essential features of structure and disorder-induced changes in electronic and optical structure are exhibited in the sp(3)s* results by two characterization parameters: the subband energy spacing and the density of states. The changes in each of them are found to depend on the interrelated trends of structure and disorder effects. (C) 2002 Elsevier Science B.V. All rights reserved.