Thermally detected optical absorption (TDOA) and photoluminescence (PL) experiments were performed at 0.35 and 4 K, respectively, on InxGa1-xN (0 < x < 0.12) layers grown on GaN-coated sapphire substrates by molecular beam epitaxy. By modelling the absorption spectra of (In,Ga)N and GaN it is possible to deduce the bandgap energy and absorption coefficient of the alloy. With the complex GaN refractive index, measured by ellipsometry, the inferences that appear in the TDOA spectra can be modelled. An approach is proposed to remove these oscillations which are also present in the PL spectra. (C) 2001 Elsevier Science B.V. All rights reserved.