This method is for determining UOP No. 5® or other p-phenylenediamine type inhibitors in gasoline. It is not applicable to aminophenolic or phenolic inhibitors. The method is limited to freshly-inhibited fuels since inhibitors that have been oxidized beyond an intermediate stage are not determined. Common gasoline dyes and tetraethyl lead do not interfere with this analysis. The lower limit of detection is approximately 0.2 mg/100 mL, which is approximately equivalent to 2.6 mass-ppm.