The experimental data on changing in the thermodynamic characteristics of lanthanides in melts of low melting metals such as Sn, Pb, Sb, Bi are obtained by the method of emf. The monotonous changing in activity and activity coefficient from La to Lu at different temperatures (660-1100 K) is observed. It enables to compute the thermodynamic characteristics in whole row of lanthanides by using the least squares method in processing the points of isothermal sections.