Abstract In Brazil, some substances are not allowed to be added to commercialised fuel, because their use would imply tax evasion, since these substances are not taxed. Therefore, monitoring the quality of commercialised fuel is a very important issue. This work presents and discusses the influence of the addition of different proportions of n-paraffin, mainly n-hexane, kerosene and diesel oil on the distillation profile and octane number of automotive gasoline. The results show that it is possible to add non-allowed substances while keeping the automotive gasoline within specification limits. This means that much research should be undertaken in order to establish methodologies that can allow inference of the presence of such substances through physicochemical analyses, even at low concentrations. Introduction Fuel, according to its type and application, require a minimum quality so that its energy can be used in the best way possible. In Brazil, this minimum quality is established through law enforceable technical specifications, aimed at insuring that all fuel comply with a minimum quality standard. However, before consumption changes may be introduced to the fuel, either through inadequate transport, handling and storage or outright adulteration, by the unlawful addition of other substances, such as solvents, kerosene or diesel. The addition of these substances to automotive gasoline is a problem that has been worrying Brazilian authorities and has prompted action from Agência Nacional do Petróleo (ANP), Brazil's petroleum regulatory agency. Fuel adulteration may not only alter its quality but also causes tax evasion. Besides, there is an environmental factor to be considered, since fuel adulteration may cause harm to the environment and is a health hazard to the consumer and the workers who handle the product, due to increased toxic gases and vapour emissions, such as carbon monoxide, nitrogen oxides and sulphur oxides. Therefore, monitoring fuel quality is of paramount importance in the fight against such irregularities, aiming at consumer protection and the establishment of healthy market conditions (1). Among the solvents most widely used for gasoline adulteration are diesel oil, kerosene and petrochemical raffinates. The large differences in the prices of gasoline, diesel, kerosene and other solvents, the easy availability of these solvents and the fact that they are miscible in gasoline, make the unhealthy and unethical practice of adulteration of gasoline a very plausible proposition (2). There is much to be gained from fraud, since the price of gasoline in Brazil is around US$ 0.90/liter, while the average price of solvents is situated around US$ 0.40/liter.