Electrophysical methods of determining the octane number of motor fuels are examined: nuclear magnetic resonance and dielectric methods. The theoretical premises for use of these methods are confirmed by the experimental data. The motor[1] and motorless (research) [2, 3] methods widely used for determining the octane number (ON) of motor fuels have a number of drawbacks: bulkiness of equipment, cyclicity of procedures, important preparation time, and low accuracy of measurements. From the point of view of operative quality control of fuels in continuous production conditions, improving the motorless method is of the greatest interest. The main problem in controlling industrial processes for manufacturing fuel with a given ON consists of ensuring the reliability and effectiveness of the information obtained. Laboratory methods in which the analysis lasts for 5-6 h bear almost no such information [4].