The osmotic/activity coefficients are one of the most fundamental and important properties of electrolyte solutions. There is currently no reliable means of predicting them from first principles without relying on extensive fitting to experimental measure- ments. The alkali hydroxide aqueous electrolytes are a particularly important class of solutions due to the crucial role they play in a vast range of applications. Here, for the first time we predict the osmotic/activity coefficients of these solutions without any fitting using a previously developed continuum solvent model of ion–ion interactions with no modifications. The feasibility of making these predictions with first princi- ples molecular simulation is also assessed. This demonstrates the reliability of this continuum solvent model and provides a plausible pathway to the fast and accurate prediction of these important properties for a wide range of electrolyte solutions.