The reactivities of novel heterocyclic diarylamine radical-trapping antioxidants (RTAs) are profiled in a heavy hydrocarbon at 160 degrees C, conditions representative of those at which diphenylamine RTAs are used industrially. While carboxylic acids produced during the autoxidation are shown to deactivate these more basic RTAs, the addition of a sacrificial base leads to efficacies that are unprecedented in the decades of academic and industrial research in this area.