A series ofInxAl1−xAssamples(0.51<x<0.55)coherently grown on InP was studied in order to measure the band-gap energy of the lattice matched composition. As the substrate is opaque to the relevant photon energies, a method is developed to calculate the optical absorption coefficient from the photoluminescence excitation spectra. The effect of strain on the band-gap energy has been taken into account. Forx=0.532,at 14 K we have obtainedEg0=1549±6 meV.©1997 American Institute of Physics.