A chemical vapour deposition process enhanced by a flowing microwave Ar-BCl3 post-discharge has been carried out to synthesise at 733 K thin films containing zirconium diboride. In this process, zirconium atoms diffuse from the substrate (Zircaloy-4) to the gas/film interface and react with boron-containing molecules to synthesise ZrB2. Secondary neutral mass spectroscopy analyses show that these films contain oxygen also. Electron probe microanalysis results are not consistent with the possible defined compounds of the Zr-B-O system described in the literature. Thus, coatings obtained by post-discharge treatment are made of several phases, which are mixed: ZrB2, B2O3 and ZrO2. This point is confirmed by X-ray photoelectron spectroscopy. However, electron diffraction analyses show that only ZrB2 is crystallised. Thus, these films are composed of ZrB2 nanograins embedded in an amorphous solid solution of zirconium and boron oxides. These nanocomposite films contain Zr(Fe,Cr)(2) precipitates also, i.e. the same than those present in zircaloy-4 substrates. However, the precipitates size in the film is about one order of magnitude higher than in substrates. According to the growth mechanism of the nanocomposite films, we present a possible explanation on the occurrence and on the size of the Zr(Fe,Cr)(2) precipitates.