Catalytic oxidation of mercaptan in petroleum distillate including quaternary ammonium hydroxide

A process for treating a mercaptan-containing sour petroleum distillate which comprises passing said distillate in admixture with an oxidizing agent through a fixed bed of a supported mercaptan oxidation catalyst in the presence of a quaternary ammonium hydroxide having the structural formula ##STR3## wherein R is a hydrocarbon radical containing up to about 20 carbon atoms and selected from the group consisting of alkyl, cycloalkyl, aryl, aralkyl and alkaryl, R.sub.1 is a substantially straight-chain alkyl radical containing from about 5 to about 20 carbon atoms, and R.sub.2 is selected from the group consisting of aryl, aralkyl and alkaryl.