In order to study the influence of yttrium on tritium behaviour in Li-Pb liquid alloys the coordination cluster theory was extended on the fourth-component systems. Using the developed approach the yttrium dependence on the thermodynamic activity of tritium in the whole concentration range of Li-Pb system were studied at 400–800°C. The yttrium displaces the concentration boundary which separates the regions with positive and negative deviations from ideality. Also, the yttrium addition has influence on threshold lead content, corresponding to conversation of the alloy from an exothermic to endothermic tritium occluder.