Amorphous carbons on tungsten oxide (a-C/WOX) bi-layers were deposited on silicon substrates by rf sputtering. The WOX layers were obtained from a pure tungsten target, in a gas mixture of argon and oxygen, whereas those of a-C were obtained from a graphite target, in pure argon plasma. The reactivity of the a-C/WOX interface and the ion bombardment effects have been studied by Optical Emission Spectroscopy (OES) technique. The OES spectra show that the a-C/WOX interface is reactive in accordance with previous results obtained by X-ray reflectometry. The inter-diffusion depth is estimated to be greater than 56 nm. Emission lines of W are still observed even after an ion bombardment time of about 45 minutes. This result confirms the presence of the Knock-on effect.