Density data for dilute aqueous solutions of benzoic acid and three hydroxybenzoic acids obtained in the temperature range 298.15 K to 498.15 K and at either atmospheric pressure or at pressures close to the saturated vapour pressure of water and at elevated pressures up to 30 MPa are presented along with partial molar volumes calculated from the experimental data. The data were obtained by using both a high-temperature high-pressure flow vibrating-tube densimeter and a commercial vibrating-tube cell DMA 602HT.