Boron nitride (BN) thin films deposited by isopressure and isothermal chemical vapour infiltration (ICVI) from BCl3-NH3-N-2 mixtures have been characterized from a physicochemical point of view as functions of both the deposition conditions and the destabilizing action of moisture. As-deposited (deposited at 773 K and post-treated at 1273 K), the BN films are turbostractic (d(002)=0.36 nm, L(c)=1.5 nm), have a low density (1.4 g cm(-3)) and contain oxygen (about 20 at %). A first oxygen content (191.5 eV by XPS) is inserted in the films during the CVI step in relation to the hygroscopy of intermediate solid products and the quasi-equilibrium between the formation of BN and B2O3. A second oxygen content (192.5 eV) is due to the hydrolysis of BN by moisture which induces a very drastic transformation of BN. This destabilization affects both boron and nitrogen atoms and leads to the formation of ammonium berate hydrates. Different post-treatments have been investigated to stabilize the BN films and it appears that nitriding under ammonia is the most efficient.