Abstract Activity coefficients for HBr in (hydrogen bromide + calcium bromide + water) have been measured at constant total ionic strengths of 0.25, 0.5, 1.0, 2.5, 3.5, 4.0, and 5.0 mol·kg −1. The temperatures covered in this study were 278.15, 283.15, 298.15, 308.15, and 318.15 K. The e.m.f. method using a cell of the type: Pt/H 2(g, p = 101.325 kPa)/HBr( m 1), CaBr 2( m 2)/AgBr(s)/Ag(s) was used. The thermodynamic properties of this asymmetrical mixture including the activity coefficients of calcium bromide in the mixture have been calculated and interpreted in terms of Pitzer's formalism. The Pitzer mixing parameters: Θ H, Ca S and Ψ H, Ca, Br (including higher-order electrostatic effects), and Θ H, Ca and Ψ H, Ca, Br (excluding higher-order effects) as well as the temperature derivatives of the mixing coefficients: ∂Θ H, Ca S ∂T and ∂Ψ H, Ca, Br ∂T , have been estimated. The temperature derivatives of the Pitzer pure-electrolyte parameters for CaBr 2 are also included.