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Sol-gel derived high temperature diffusion barrier coatings are applied on CMC substrate (C/SiC) in order to seal the porous material by coating and prove its gas permeability. Inorganic and hybrid sol-gel systems are developed resulting in mixed oxides such as the Al 2 O 3 -SiO 2 system (mullite) and the ZrO 2 -SiO 2 system. The sol-gel systems are characterized by FT-IR, ATR, TGA, DSC, XRD, and viscosity measurements. The investigations include studies on film forming techniques. Samples are coated by infiltrated coating process involving the use of a specific developed infiltration device, by dip coating, as well as combined coating processes. Coated samples are dried at room temperature and sintered at 1300 ºC in inert atmosphere. Beside weight gain monitoring, the deposited sol-gel coatings are characterized in respect of permeability indicated by flow rate across the samples. Furthermore, coating analysis are SEM, EDX, XRD, and TGA. Inorganic infiltrated multi layers (11 coating steps) exhibit flow reduction of 53 %. Dip coated hybrid sol-gel coatings reduce the flow rate across the C/SiC samples about 50 % by a single coating step. * TEOS: tetraethyl ortosilicate, AlB: aluminiumsec-butoxide, AcAc: acetylacetone Isopr: isopropanol, HCl: hydrochloric acid, ZP: zirconium (IV)propoxide