AbstractThe equilibrium acidities of phenylacetonitrile, and 20 of itsm−andp‐substituted derivatives have been measured in Me2SO solution. Their pKa′s plot linearly with those of the corresponding anilines. Combination of the pKa′s of these acids with their oxidation potentials,Eox(HA), and those of their conjugate bases,Eox(A−), provide an estimate of the acidities of the corresponding radical cations. The pKHA+values for ArCH2CN+., where Ar is Ph, 1‐ and 2‐naphthyl, and 9‐anthryl, are −32, −18·5, −17·5, and −11, respectively, compared to 21·9, 20·85, 20·65, and 19·8 for the corresponding ArCH2CN acids. Acidities of PhCH(Me)CN+., Ph2CHCN+., 9‐CN‐FlH+., and 9‐CN‐XnH+.are −33, −35, −25, and −27, respectively, compared to 23·0, 17·5, 8·3, and 13·6 for the corresponding acids from which they were derived. The homolytic bond dissociation energies (BDEs) for the benzylic CH bonds in these arylacetonitriles, estimated by combining pKHAwithEox(A−), fall in the range of 69 kcal/mol for 9‐CN‐XnH to 82 kcal/mol for PhCH2CN. For GC6H4CH2CN+.radical cations the acidities are decreased, relative to G=H, when G is an electron donor substituent and increased when G is an acceptor. The BDEs of the benzylic CH bonds in GC6H4CH2CN are weakened by up to 4 kcal/mol byparadonors and strengthened by up to 1·2 kcal/mol bym‐ orp‐acceptors. The significance of these changes in BDEs with regard to the use of σ.scales and the ΔAOP method for