High-resolution X-ray diffraction (HR-XRD) with rocking curve, atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy have been performed on highquality quaternary Al x In y Ga 1 À x À y N thin films at room temperature. The Al x In y Ga 1 À x À y N films were grown on c-plane (0 0 0 1) sapphire substrates with AlN as buffer layers using a molecular beam epitaxy (MBE) technique with aluminum (Al) mole fractions x ranging from 0.0 to 0.2 and constant indium (In) mole fraction y= 0.1. HR-XRD measurements confirmed the high crystalline quality of these alloys without any phase separation. The X-ray rocking curve of Al x In y Ga 1 À x À y N films typically shows full widths at half maximum (FWHM) intensity between 14.4 and 28.8 arcmin. AFM measurements revealed a two-dimensional (2D) growth mode with a smooth surface morphology of quaternary epilayers. PL spectra exhibited both an enhancement of the integrated intensity and an increasing blueshift with increased Al content with reference to the ternary sample In 0.1 Ga 0.90 N. Both effects arise from Al-enhanced exciton localization. PL was used to determine the behavior of the energy band gap of the quaternary films, which was found to increase with increasing Al composition from 0.05 to 0.2. This trend is expected since the incorporation of Al increases the energy band gap of ternary In 0.1 Ga 0.90 N (3.004 eV). We have also investigated the bowing parameter for the variation of energy band gaps and found it to be very sensitive on the Al composition. A value of b = 10.4 has been obtained for our quaternary Al x In y Ga 1 À x À y N alloys.