The apparent pKa values for the acids (I) and (II), where R is Me, Et, Pr1 and But, respectively, and for benzoic acid, have been determined by potentiometric titration in 30 per cent v/v acetone-water. The choice of this solvent system was dictated largely by the lower solubilities of the acids (II) compared with those of (I). Each alkoxybenzoic acid (I) is weaker than its alkylthio analogue (II); this is consistent with the more limited data of Baker, Barrett and Tweed3. Relative to benzoic acid, the variation in acid strength with variation in R is for (I), Benzoic>But>Me>Et>Pr, and for (II), But>pri>Benzoic>Et>Me. These trends confirm those observed by Cavill, Gibson and Nyholm2 for (I; J? = Me, Et, and Pri), and by Baliah, Shanmuga-nathan and Varadachari4 for (II; R = Me, Et, Pri). The position of the But acid in each case is a clear demonstration of the operation of a steric effect involving the But group and the o-hydrogen atoms, which prevents the attainment of coplanarity of the bonds of the oxygen (or sulphur) atom and the benzene ring, and thereby destabilizes the acid relative to its anion. It would appear that a small, though real, steric effect operates in (II) even when R is Pr1. The steric interactions present in (II; i? = Pri and But, respectively) would appear to be so effective as to hinder the operation of the electron-releasing mesomeric effect of the sulphur atom to such a degree that the electron-withdrawing inductive effect of this atom becomes predominant in determining the free-energy difference between each of these acids and its anion. A more complete interpretation must await the outcome of investigations into the esters of (I) and (II), and the corresponding ra-alkoxy- and alkylthio-benzoic acids, and their esters.