For technical applications of supported ionic liquids (ILs), the stability of the IL layers both with regard to thermal decomposition and to losses by evaporation is of great importance. An innovative radio frequency-based method is presented to determine the pore filling degree of supported ILs in situ and in a contactless way. As an example, the IL 1-ethyl-3-methylimida zolium bis(trifluoromethylsulfonyl) imide, which was supported on the internal surface of porous silica, was selected. The complex permittivity of the porous solid coated with the IL increases linearly with the IL pore filling degree. Therefore, the evaporation rate of the IL in a fixed bed could be measured in situ in the reactor.