AlxGa1-xN epitaxial films were grown by gas source molecular beam epitaxy and investigated as a function of Al mole fraction. Cathodoluminescence, photoluminescence, and optical absorption measurements were used to characterize 1 mum thick layers of AlxGa1-xN with nominal x values of 0.1, 0.1, 0.3, 0.4, and 0.5 as well as GaN and the AlN buffer layer. The GaN and AlxGa1-xN layers were doped with 1 x 10(18) cm(-3) of Si from a Knudsen source. Typical spectra contain a donor bound exciton peak with its phonon replicas and donor-acceptor pair (DAP) peaks. From the observed bound exciton peak positions and the absorption data, the band gap energies for the AlxGa1-xN were estimated. and these values were compared with the linearly extrapolated band gaps. This study indicates that the MBE growth of 1 mum thick AlxGa1-xN layers provides good quality films for x values up to 0.3, and lesser quality films for x values above this mole fraction, requiring further improvement and,or modifications to the MBE growth technique to obtain quality AlxGa1-xN alloy material for semiconductor devices. Published by Elsevier Science B.V.