**Abstract**

The quality of automotive fuels—gasoline, diesel, biodiesel, and ethanol—is discussed in the light of their specifications in different countries and regions and in terms of their adulteration, which has such a prejudicial effect on the production chain, distribution chain, tax revenues, the environment, and end consumers. Different ways of adulterating automotive fuels are analyzed, as are the procedures for their detection. Several analytical methods for monitoring quality and detecting adulteration have been addressed in the literature, emphasizing the determination of properties such as density, distillation curve, octane rating, vapor pressure, etc., by means of physicochemical methods and chromatographic and spectrographic techniques, to the detriment of colorimetric methods. This chapter looks at colorimetric techniques designed for quality monitoring and the detection of adulteration in fuels, especially simple, quick, low-cost procedures with potential to be used in the field.

**Keywords:** quality, adulteration, gasoline, diesel, biodiesel, ethanol, methanol, glycerin, water, colorimetric tests, hydroxamic acid test, Schiff's reagent, solvatochromic effect, field test
