The possibility, in principle, of determining the content of undissolved water in fuels with high precision using high-quality super-high frequency (SHF) resonance systems (microwave cavity resonator) is substantiated. Results of experimental study of a cylindrical cavity resonator with a thin layer of water on the bottom-end wall are adduced. The proposed method is analyzed using simulation by the finite-element method in the ANSYS system. Results of this research can be of practical use for determining deposited water in jet fuels.