The temperature dependent and concentration asymmetric interaction parameter in the liquid phase region is computed from reliable thermodynamic data and phase-boundary composition by treating the single liquid phase of Cs-Te as a sub-regular solution. The excess Gibbs energy of the liquid phase of Cs-Te is determined to be G L E =x C s xx T e {(-289-0.00144xT)x T e +(-972+0.482713xT)x C s } kJ/mol in the temperature range 900-1100 K. The interaction parameter so deduced is used to calculate the activities of Te and Cs in the single liquid phase and two-phase regions around Cs 2 Te.