The thermodynamic properties of aqueous NaOH are represented for the entire range to extremely high concentration by mole fraction based equations. The apparent molar volume and the solvent and solute activities are obtained from literature values based on experimental measurements extending over the ranges 273-523 K and 0-400 MPa. For the range below 6 mol kg(-1) the values from the very comprehensive treatment of Simonson et al. are given high weight. At the highest temperature the dissociated mole fraction of NaOH extends to 0.9. The equations represent the data satisfactorily over this very wide range and are of a form readily extended to more complex mixed systems and to solubility calculations.