It has been postulated that mitochondrially derived reactive oxygen species (ROS) play a major causative role in aging processes. The primary sources of these oxidants are believed to be complexes I and III of the electron transport chain, with little evidence supporting oxidant formation at complex II (succinate dehydrogenase). Mutation of a complex II protein has, however, been shown to cause increased oxidative stress and decreased life expectancy in the Caenorhabditis elegans mutant mev-1. A recent study by Yankovskaya and colleagues, in which the structure of Escherichia coli succinate dehydrogenase was determined, provides an explanation for these observations. Furthermore, these results suggest possible mechanisms by which electron leakage might occur at this site in the aged organism.