Ortho-xylene has been shown to possess a much greater degree of reactivity towards oxygen than its isomers meta- and para-xylene. Under identical conditions, it exhibited a maximum reaction rate of the order of ten times that observed for the less reactive isomers. Likewise, a more pronounced acceleration period occurs, suggestive of a more vigorous branching agent than formaldehyde. Analytical results have indicated a relation between reaction rate and the concentration of the product phthalan. A reaction mechanism is presented which illustrates the importance of intramolecular isomerization steps, and successfully explains most of the phenomena associated with the combustion of this isomer. Comparisons are drawn between the three xylenes on the basis of suggested mechanisms.