Gasoline is a volatile, inflammable liquid composed of a mixture of paraffinic, naphthenic, olefinic, and aromatic hydrocarbons (C-4-C-12). During storage, these components react with atmosphere oxygen and with each other, promoting changes in their physical-chemical characteristics due to the fort-nation of gums. These materials have polymeric character and normally cause unwanted effects such as deposition in car filters and distribution lines. A discussion of olefin types, their contents, and the oxidation reaction mechanisms in Brazilian gasolines is presented in this work. Doped gasolines subjected to oxidation were analyzed for density, induction period, and gum content. The results show that not all olefins contribute positively to gum formation. Among the olefins studied, those which form secondary allylic radicals (cyclohexene and 2,4-hexadiene) are the ones which most contribute to the formation of gum. (c) 2006 Elsevier Ltd. All rights reserved.