Optical properties of zinc oxynitride (ZnxOyNz) films are investigated. ZnxNyOz films are deposited onto glass substrates from a ZnO target in N-2-Ar mixtures by reactive rf magnetron sputtering. Structure and chemical bonding states are studied by X-ray diffraction and X-ray photoelectron spectroscopy, respectively. Transmission spectra are measured with a double beam spectrometer. Absorption coefficients are calculated from the transmission spectra with film thickness, and their dependence on photon energy is examined to determine optical band gap (E-g). E-g decreases from 3.26 to 2.30 eV with increasing nitrogen concentration in the films. ZnxNyOz is a promising optically functional material that can act in the visible range.