The stabilities and electronic structures of the bulk and several low-index surfaces of beta-TaON were studied quantum-chemically at semi-empirical and hybrid density-functional level. Excitation spectra of the bulk and surfaces were calculated by using an iterative CIS approach (MSINDO-CIS). The calculated structure parameters, atomic positions and band gaps for the bulk are compared with experimental data. For comparison we also studied a hypothetical configuration of TaON where the anion ordering is changed. The O/N distribution has a significant effect on the optical spectra and may effect the photocatalytic efficiency of the material. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim