The densities, heat capacities, heats of dilution, osmotic coefficients, viscosities, and conductivities of sodium benzoate and the ortho, meta, and para isomers of sodium hydroxybenzoate have been measured in water at 25 °C. The densities and heat capacities of phenol solutions and the osmotic coefficients of aqueous potassium benzoate have also been determined. The addition of an —OH or a —COONa group on a benzene ring has little effect on the properties related to the effective size of the solute (apparent molal volume, Bη viscosity coefficient, and ionic conductivity) but decreases significantly the apparent molal heat capacity. The addition of an —OH group in the meta or para position of sodium benzoate has a similar effect. The large negative contribution to the heat capacity probably reflects the solute–solvent hydrogen bonding. The ortho isomer, which can form an internal hydrogen bond, has a significantly different behavior from that of the other isomers.The excess properties show no evidence of association at low concentrations although some specific interaction is apparent for the ortho isomer at high concentration. There seems to be some cation–anion structural attraction with the meta and para isomers which again may be related to the hydrogen-bonding ability of these solutes. Sodium and potassium benzoates show signs of association above 1 m.