The basicities of 15 ortho-substituted benzoic acids have been measured in sulphuric acid. The ortho effects of alkyl groups are base weakening and increase in the expected order methyl, ethyl,i-propyl,t-butyl. Ortho halo benzoic acids have comparable basicities and are weaker than their para isomers. Oxygen-containing ortho substituents, except hydroxyl, all cause substantial base strengthening relative to the para isomers. Even the groups nitro and carboxyl when substituted ortho in benzoic acid cause the basicity to rise, presumably because of hydrogen bonding in the conjugate acids.