We have optically characterized a series of GaAs1-xSbx epilayers (0.19<x<0.71) grown by molecular bean epitaxy on semi-insulating GaAs substrates, with surface orientations of (001), (001) 8degrees toward (111)B, (001) 8degrees toward (111)A, and (115)B. For each of these samples, we have investigated the absorption as a function of temperature (4 K<T<300 K) using Fourier transform infrared spectroscopy techniques. The band gap at each temperature was determined from the photon energy dependence of the absorption coefficient and compared with theoretical predictions. From our results we have obtained the Varshni coefficients, alpha=(4.2+/-0.1)x10(-4) eV/K and beta=(189+/-9) K, which describe well not only the temperature dependence of the band gap for the entire alloy range of our samples, but also for the past experimental work of others. These values differ significantly from what we believe are the only other reported values by K. G. Merkel [K. G. Merkel , Appl. Phys. Lett. 65, 2442 (1994)]. (C) 2002 American Institute of Physics.