<p num="65">FIELD: measuring equipment.</p> <p num="66">SUBSTANCE: invention relates to the measurement equipment and can be used for the fuel octane characteristics current monitoring. For the method implementing, the two samples are taken, one of which is the commercial gasoline with the high-octane component fixed volume fraction therein, taken on the its production process line, and the second sample with the commercial gasoline high-octane component with its octane number fixed rated value is taken on its supply line for compounding, measuring these samples optical density spectra in the wide range of the mid-IR range and determining their difference spectrum, according to which determining the first sample low-octane component octane number from the previously constructed multidimensional calibration characteristic, after which, summing up these samples low-octane and high-octane components octane numbers, taking into account of their volume fractions in the commercial gasoline, and by this sum value