A test method for the quantification of N-methylaniline (NMA) in a hydrocarbon medium has been developed. The method is based on obtaining a colored test species on the surface of indicator tablets and subsequent recording of the analytical signal using colorimetry. The possibility of determination of NMA in the gasoline fraction has been shown. The metrological characteristics of the method have been calculated. The detection limit of the method for NMA in hydrocarbon media is about 0.014 vol %. It has been shown that the fuel components aromatic hydrocarbons and nitrogen and sulfur hetero compounds do not interfere with determination of NMA in a hydrocarbon medium.