Rate coefficients at 30 °C have been measured for the reactions of benzoic acid and a large series of ortho-sub-stituted benzoic acids with diazodiphenylmethane (DDM) in 7 aprotic solvents. The dependence of the second-order rate coefficents on the initial concentration of the acid has been examined for some of the systems. For reactions in a given aprotic solvent the log k values for the various acids (determined as far as possible with standard [acid]) were correlated by means of the appropriate form of the extended Hammett equation with inductive, resonance, and steric parameters. The correlation equations were established with a limited set of substituents, whose characteristic parameters σ_I, σ_R, and υ(as determined by Charton) are well established and are unlikely to show effects due to hydrogen-bonding or substituent conformation. The results for other substituents were then interpreted by comparing values of log k(calc.) from a correlation expression with log k(obs.). The most striking finding from the analysis is the very great importance in aprotic solvents of effects involving hydrogen-bonding between ortho-substituent and reaction centre.