This method is for determining the purity of UOP No. 5 inhibitor (N,N'-di-sec-butyl-p-phenylenediamine). The method is applicable to high purity UOP No. 5 and to production stream samples. The following components and group classifications are reported: light ends, methyl ethyl ketone (MEK) and methyl ethyl carbinol (MEC), light boiling intermediates, monoalkylated UOP No. 5, heavy boiling intermediates, UOP No. 5, trialkylated UOP No. 5, and heavies. MEK and MEC co-elute and are reported as a composite. Components that do not elute from the chromatographic system, if any, under the specified conditions, commonly called tars, are not determined. The identified components are reported on a tar-free basis. The range of quantitation for any single component is 0.01 to 99.99 mass-%.