NMR shifts of N-phenyl-N'-alkyl-p-phenylenediamines (PPD) in DMSO have been measured as well as evaluated by B3LYP calculations. According to Simon et al. Molar Antioxidant Effectiveness (AEM) of PPD antioxidants depends on the bond strength of hydrogens to amine nitrogens between aromatic rings (N-A), to the side aliphatic chain nitrogens (N-B) and to its neighboring tertiary carbon atoms (C-T). AEM increases with NMR shifts of H-A, H-B, N-A and probably also of C-T atoms whereas NMR shifts of N-B atoms exhibit a reverse trend. This is very surprising because similar reactions at A and B sites are supposed. (C) 2015 Elsevier B.V. All rights reserved.