Acidity constants at 20° and ionic strength μ = 0.1 were determined for the following substances: BzOH (3.99), terephthalic acid (-, 4.34), PhOH (9.80), p-nitrophenol (6.98), salicylic acid (2.69, 12.3), m-hydroxybenzoic acid (3.69, 9.61), and p-hydroxybenzoic acid (4.36, 8.985).The pK values are discussed in terms of Hammett's rule, and a σ*-value (for use with reactions of phenols and anilines) is calculated for p-COO-: σ* = 0.351 ± 0.016.Cf. Jaffe, C.A. 48, 1120d.