A method for treating a spent caustic solution having a pH greater than about 10.5 includes the steps of (a) lowering the pH of the caustic solution into the range of between about 10 to 10.5 to produce a pH-modified caustic solution; (b) oxidizing the pH-modified caustic solution until the solution to produce a non-sulfide reactive oxidized solution; and (c) lowering the pH of the oxidized solution to a pH less than 9. Methods for oxidizing inorganic substrates in aqueous solutions using an oxidizing agent such as pure oxygen, oxygen-enriched air or ozone are improved by including in the headspace above the aqueous solution a diluent gas and by recycling the oxygen with diluent gas into the reactor.