The densities of aqueous solutions of 4-methylphenol and its mixtures with sodium hydroxide were measured as a function of concentration at T = 298 K at atmospheric pressure, and between the temperatures 373 K and 463 K at a pressure of 2 MPa. The partial molar volume at infinite dilution of 4-methylphenol (HCRES) and its sodium salt (NaCRES) were determined by the application of the Pitzer ion-interaction treatment to the solutions neglecting high order terms. The reported values of partial molar volume of 4-methylphenol are in good agreement with literature data. (C) 2003 Elsevier Ltd. All rights reserved.