The thermodynamic factor in diffusion in the Ll2-type non-stoichiometric intermetallic compounds Ni3Ga and Pt3In was derived from the experimentally measured activities of Ga and In, respectively. The results show that the thermodynamic factor in diffusion in the Ni3Ga and Pt3In compounds exhibits the maximum near the stoichiometric composition with the variation of composition over the compound region. The values indicate strong interaction between Ni and Ga atoms, as well as Pt and In atoms, respectively, as expected in ordered alloys. The thermodynamic factor in diffusion in the Ll2-type non-stoichiometric intermetallic compounds Ni3Ga and Pt3In was derived from the experimentally measured activities of Ga and In, respectively. The results show that the thermodynamic factor in diffusion in the Ni3Ga and Pt3In compounds exhibits the maximum near the stoichiometric composition with the variation of composition over the compound region. The values ​​indicate strong interaction between Ni and Ga atoms, as well as Pt and In atoms, respectively, as expected in ordered alloys.