Abstract Mean molal activity coefficients and water activities of rare earth chlorides and bromides at 25 °C were determined in aqueous solutions from dilute to concentrated. The mean molal activity coefficients of the rare earth bromides are larger than those of the chlorides, whereas the water activities of the bromides are smaller than those of the chlorides. These trends are discussed in terms of the ionic radii of the catonic and anionic species, ionic interactions and hydration. The osmotic coefficients of the rare earth bromides are fitted to the Pitzer equation. The Pitzer parameters for the rare earth bromides were evaluated by comparison with those for the chlorides.