Data have been compiled for the pK, values in water at 25°C of about 60 meta- or para-substituted benzoic acids, with a view to proposing recommended values of Hammett substituent constants, σ. The number of substituents covered is about one-half of this number, but for certain substituents data are not available for both the meta and the para position. The amount and the reliability of the data vary greatly from one acid to another. Critical appraisal leads to the classification of the substituents and their σ values as belonging to a First Division (with more reliable data) or a Second Division (with less reliable data). The substituents in the former category are: Me, NO₂, OMe, F, Cl, and Br (each in both the meta and the para position). Recommended values of σ are tabulated for substituents in each Division, but for certain members of the Second Division there are caveats because the data show discrepancies.