Relations have been established between the pKa values for the non- zwitterionic forms of meta- and para-aminobenzoic acids, pKB, and the pKa values of the corresponding methyl and ethyl esters, pKE. For methyl m-aminobenzoate, the expression is pKB = pKE + 0.09, whilst for the ethyl ester it is pKB = pKE + 0.03. The corresponding relations for p- aminobenzoic acid derivatives are pKB = pKE + 0.06 for the methyl ester, and pKB = pKE + 0.03 for the ethyl ester. The mioroscopic constants pertaining to the zwitterionic equilibria are given, and are used to establish the Hammett substituent constants, sm, for the following meta substituent groups: NH2, 0.00; NH3+, 0.98; NHCH2COOEt, -0.10; COOH, 0.30; COOMe, 0.33; COOEt, 0.31; COO-, 0.01.