The formation of a secondary inhibitor during the inhibition of liquid-phase radical oxidation with high-molecular heteroatomic compounds of petroleum was studied by microcalorimetry. The resinous-asphalt-enic materials of different origin were investigated. It was shown that new structures with phenol and quinone groups, which play the role of a secondary inhibitor, were produced by the action of peroxide radicals in the model reaction of radical oxidation of petroleum products. Based on a difference between the heat-release intensity in the initiated and inhibited oxidation, a relationship for calculating the secondary-inhibition rate constants (k(y)) was derived. The values for the rate constant ky were found to be substantially lower than the inhibition rate constants of the initial petroleum inhibitor.