Thin films in the boron-carbon system have been deposited by CO2 laser chemical vapour deposition (LCVD) on fused silica plates from a gas mixture of BCl3, CH4, H-2 and Ar. Glancing incidence X-ray diffraction (GIXRD) and electron-probe microanalysis (EPMA) were extensively used for characterisation of structure and chemical composition, respectively. Depending on the process parameters, three different phases were deposited : rhombohedric boron carbide and graphite (stable phases), and a metastable tetragonal boron rich carbide. Thermodynamic calculations were performed for a broad range of gas phase compositions and the data obtained were compared to the experimental results. This comparison showed a generalised lack of carbon (or excess of boron) in all the deposited films and was also used to estimate the deposition temperature.