[en] Highlights: • Thermodynamic activity measurements of the main constituents in industrial alloys. • Measurements performed in 1483–1593 K temperature range. • Comparison of experimental activities to calculated one using a commercial database. • Good agreement for Cr, Ni and Fe results but a larger discrepancy for Cu. - Abstract: The first activity measurements of nickel, copper, chromium and iron in industrial nickel-base alloys (Monel®400, Monel®K-500, Nicorros®400, Nicorros®K-500, Hastelloy-X®, Hastelloy-C2000®, model alloy 1181) and steels (Uranus®65 and Uranus®45N) were performed using multiple Knudsen effusion Cell Mass Spectrometry in the temperature range 1483–1593 K. For each constituent, the activity is expressed as: ln(a_i) = (A ± δA)/T + (B ± δB). The experimental results were compared with calculated data using the SSOL2 database from Thermo-Calc.