One of the conditions for ensuring the safety of air transport operation is the quality of aviation fuel refueled in aircraft. Fuel quality control is a multi-parameter task that includes monitoring the free moisture content. Regulatory documents establish the free water content of not more than 0.0015 wt.%. A direct electrometric microwave resonator method for controlling free moisture in aviation fuels has been developed. The method consists in measuring the resonance characteristics of a cylindrical cavity resonator. At that, on a solid dielectric surface inside the resonator, there are moisture drops that are pressed by a thin plate. As a result, dielectric losses increase dramatically. Analytical and experimental analyses of the proposed method are carried out. The monitoring range of 0.5–30 μl of the absolute moisture volume in aviation fuels with a maximum error of no more than 25% is justified. The sensitivity of the proposed method for monitoring microwave losses in free moisture drops transformed into a thin layer by pressing with a thin plate is an order of magnitude higher than the sensitivity of the method for monitoring microwave losses in moisture drops on a solid surface in a resonator. The proposed method can be used as a basis for the development of instruments for monitoring the free moisture of aviation fuels in airfi eld and laboratory conditions. The direction of development of the method is shown.