The reactions that lead to fuel instability are not fully understood. Various reports in the literature link particular fuels with specific reactions, usually oxidation or free-radical processes. Trace quantities of metals are present in all processed fuels. The metal source can be naturally occurring or present from fuel handling. The usual case is to look at the catalysis reactions of metals or metal ions with molecular oxygen or other active oxygen species such as hydroperoxides. This paper reports on the reaction of a nitrogen heterocycle, indole, with Ni(0). We propose a mechanism that shows that catalytic amounts of metals such as nickel can produce compounds of higher molecular weight and increased polarity and thus actively participate in the fuel-instability process.