The thermodynamic properties of Ba-Pb alloys (activities, phase transitions, and partial molar quantities of Gibbs energy, entropy and enthalpy) were determined based on electromotive force (emf) measurements for thirteen mole fractions (x(Ba) = 0.05-0.76) from 700-1060 K using an electrochemical cell: Ba(s) vertical bar CaF2-BaF2 vertical bar Ba(in Pb). The activity of Ba in liquid Pb was found to be as low as 2.0 x 10(-10) at x(Ba) = 0.05 and 973 K, reflecting a large excess partial molar Gibbs energy of -156 kJ mol(-1) due to strong chemical interactions between Ba and Pb. Emf measurements were most reliable in the presence of a liquid phase, but were less reliable for high-melting compositions due to the formation of metastable phases during thermal cycling, confirmed by thermal and structural analyses using differential scanning calorimetry and X-ray diffraction. The experimentally-determined activity values of Ba-Pb alloys provide a new, reliable data set for further refinement in the assessment of thermodynamic properties for the Ba-Pb system. (C) The Author(s) 2019. Published by ECS.