The study of tritium dissolved in liquid lithium systems containing metallic impurities is of technological interest for tritium removal processes and suppression of the tritium leakages from the blanket in a fusion reactor. The interaction parameter formalism and coordination cluster theory have been used to calculate the tritium activity coefficients in Li-T-Al, Li-T-Mg, Li-T-Si, Li-T-Y and Li-T-La systems. Calculations performed demonstrated that silicon, aluminium and magnesium have no influence on the tritium activity coefficients in dilute lithium solutions within the temperature range 400–800°C, but yttrium and lanthanum may be used to effectively decrease this coefficient in lithium melts.