The instant invention discloses a method of removing heterocyclic basic nitrogen compounds from petroleum oils which comprises first enhancing the concentration of the heterocyclic basic nitrogen compounds in a portion of the petroleum oil and then extracting the enhanced portion of the heterocyclic basic nitrogen compounds with a two phase system having an aliphatic carboxylic acid as the extracting agent. In the hydrotreating embodiment of this invention a portion of all of the nitrogen compounds are converted to heterocyclic basic nitrogen compounds and thereby more feasibly extracted. In the distillation embodiment of this invention the nitrogen compounds are actually concentrated and thereby result in a more feasible extraction with the aliphatic carboxylic acid.