Tributyl phosphate and its degradation products saturated with nitric acid and exposed to elevated temperatures lead to an accidental condition known as "reactive red oil formation". The present study aims at elucidating the chemical pathway of this reaction in an Accelerating Rate Calorimeter (ARC). The thermal characteristics obtained from ARC coupled with end product analysis using spectroscopic techniques proved that red-oil forming mechanisms varied as per the concentration of nitric acid. The chemical pathway for red oil formation was found to occur through the oxidation of butanol at lower temperatures and with dilute nitric acid, the predominant path was via butyl nitrite intermediate at higher temperatures. Independent ARC experiments with butanol and butyl nitrite with nitric acid validated the mechanism. This study also revealed that most of the diluents employed for TBP undergo exothermic reaction with nitric acid, even in the absence of TBP. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.