The aqueous dissociation constants for a number of substituted picric acids and related compounds have been determined spectrophotometrically, and the values obtained correlated with the mid-equivalence potentials obtained by half-neutralization in acetone solution. Analysis of these results together with those obtained similarly for substituted benzoic acids shows that while the mid-equivalence potential method affords a rapid and convenient means of making a good estimate of the acid strength of a compound, it has its limitations and is only strictly applicable within the confines of a series of closely similar compounds. The observed pKa values of a large series of polysubstituted phenols were correlated with the predicted values obtained from the Hammett relation, and good agreement obtained, even with substituted picric acids; styphnic acids appear to behave anomalously.