The GaAsSb/GaAs conduction band alignment is determined to be almost flat (weak type-I) in the 0.3 Sb mole fraction neighborhood where 1300 nm emission is observed. Based on photoluminescence measurements and modeling the bandgap bowing parameter of pseudomorphic GaAsSb on GaAs is determined to be -2.0 eV with the bowing distributed 45% to the conduction band. Using these parameters the GaAsSb band edge energies are determined over the 0.0 to 0.5 Sb mole fraction range. The effect of band alignment on electron-hole overlap and wavelength extension is discussed. (C) 2002 Elsevier Science B.V. All rights reserved.