The activities and partial molar heats of mixing in the Fe-V system and the activities of Fe and V in the Fe-V-Cr system at 1600°C have been determined. The analysis of Knudsen cell effusates with a mass spectrometer was applied as the experimental technique. The ion current ratio for the alloy components was measured through the whole range of composition for the Fe-V system and in the range of composition, N_V≤0.3 N_Cr≤0.2, for the Fe-V-Cr system, and the thermodynamic values were calculated by the modified GIBBS-DUHEM equation. The Fe-V binary system exhibits negative deviation from ideal behaviour. The results for the Fe-V binary system can be partially represented by the equations
log γ_V =-1.02 N²_Fe+0.24 (0≤N_V≤0.20)
log γ_Fe=-0.11 N²_V-0.31 (0≤N_Fe≤0.02)
The results for the Fe-V-Cr ternary system can be represented by the equations
log γ_V=-0.42+0.70 N_Cr (N_V=0.2, 0≤N_Cr≤0.2)
log γ_Fe=-0.04-0.18 N_Cr (N_V=0.2, 0≤N_Cr≤0.2)
log γ_V=-0.28+0.48 N_Cr (N_V=0.3, 0≤N_Cr≤0.2)
log γ_Fe=-0.08-0.20 N_Cr (N_V=0.3, 0≤N_Cr≤0.2)