Abstract Activities of aluminium in aluminium–bismuth–antimony alloys have been determined over the temperature range (1173 to 1323) K by a concentration cell technique involving the chloride electrolytes. The measured activities in the system exhibit positive deviation from ideal behaviour over the entire range of composition studied. The deviation increases progressively with the Bi content of the alloys for a constant compositional path of Sb. However, the deviation decreases substantially with the increase of Sb content in the system. The proposed equation is capable of interpreting the experimental data pertaining to the liquid–liquid immiscibility in the system. The ternary interaction parameters involved in the equation are susceptible to the physico-chemical interpretation based on the central atoms model.