A HammettH0acidity function based on the protonation of 17 diphenylamines in 20 volume % ethanol – aqueous sulfuric acid has been established. TheH0value for the most acidic solution studied (11.2 Msulfuric acid) is −6.97. This acidity function differs from that based on the protonation of azobenzenes in the same solvent system; the latter diverges to more negativeH0values as the sulfuric acid concentration increases.Thevalues for the protonation of the diphenylamines vary from +1.36 for 4-methoxy-diphenylamine to − 6.21 for 4,4′-dinitrodiphenylamine. A plot ofversus the Hammettσconstants for five monosubstituted diphenylamines yields aρvalue of +3.36. Thevalues for 4-methoxy-, 4-methyl-, 4-methylsulfonyl-, and 4-nitro-diphenylamine are all less (more negative) than expected from the Hammett substituent constants. The substituent effects on the basicities of aniline and diphenylamine are the same.The basicities of several nitro-substituted diphenylamines appear to vary regularly, and do not reflect the presence of a strong interaction between the nitro group and sulfuric acid.