Optical properties of the Boron Carbon Nitride (BCN) thin films deposited in a multi gun radio frequency (rf) magnetron sputtering system using a B4C target have been studied. Films of different compositions were deposited by varying the ratio of argon and nitrogen in the sputtering ambient. The films were annealed in dry oxygen ambient in the temperature range of 300 degrees C-700 degrees C. The effect of annealing on the optical properties of the films was investigated. It was found that the optical transmission of the films increased with nitrogen incorporation. Annealing at higher temperatures leads to considerable increase in optical transmission. Optical energy gaps (Tauc gap) calculated from the absorption data are influenced by annealing temperatures. Changes in optical properties were correlated to the chemical modifications in the films due to annealing through X-ray photoelectron spectroscopy (XPS). Studies reveal that the carbon and nitrogen concentrations in the films are highly sensitive to temperature. Annealing at higher temperatures leads to broken C-N bonds which results in the loss of C and N in the films. This is believed to be the primary cause for variations in optical properties of the films. (c) 2013 Elsevier B.V. All rights reserved.