Ta-O-N thin films were deposited by reactive magnetron sputtering from a metallic Ta target in a mixed O-2/N-2/Ar atmosphere. The ratio between Ar and the reactive gas mixture in the plasma was 1:1, and the O-2/N-2 ratio varied between 0.08 and 1.33. The depositions were performed without substrate heating. The oxygen fraction f(O) = O/(O+N) in the films increases linearly with O-2/N-2 ratio in the plasma and stabilises at O-2/N-2 >= 0.75 corresponding to f(O) = 0.93-0.96. Physical and chemical properties of the Ta-O-N films strongly depend on f(O). Three groups can be distinguished: films with 1) low (0.06-0.19); 2) intermediate (0.31-0.70); and 3) high (0.80-0.96) f(O). Ta-O-N films with low f(O) have a metallic character and are opaque. Their optical properties show similar behaviour as that of Ta-N films, while the optical properties of the films with high f(O) are identical with those of insulating Ta2O5. The index of refraction of the films with intermediate f(O) decreases from 2.5 to 2.1 with f(O). Most films are poorly crystallized and film amorphisation becomes more pronounced with increasing f(O). The nanohardness decreases with f(O) and varies from 27 GPa for f(O) = 0.06 to 5 GPa for f(O) = 0.96, fluctuating around 12 GPa for the films with intermediate f(O). (c) 2006 Elsevier B.V. All rights reserved.