The impact of thermodynamic factors on deviation from linearity of diffusion path in the ternary system Cu-Fe-Ni is analyzed. For that the slope function of the diffusion path for the diffusion couples 65Ni30Cu5Fe –29.5Ni16.5Cu54Fe, 49.5Ni50.5Fe – 51Ni49Cu and 84Cu16Ni – 50Ni50Fe, annealed at 1000°C for 196h, were calculated by an approximate equation using only thermodynamic data. Results of the calculation were compared with the values of the slope function obtained directly from experimental data. It is shown that despite of the fact that the tracer diffusion coefficients of the components in the system Cu-Fe-Ni are not equal the coincidence between the calculated and experimental values of the slope function is remarkable. This allows us to conclude that at least in this case the deviation of the diffusion path from linearity depends mainly on the thermodynamic properties of the system.