The activity of oxygen in technically important nickel melts containing 15 wt pct cobalt and 5 wt pct molybdenum has been determined at 1600‡C for various concentrations of chromium ranging from 0 to 30 wt pct and aluminum varying from 0 to 15 wt pct. The activity of oxygen was measured by an electrochemical technique using yttria-doped thoria electrolyte cells. The results obtained are analyzed in terms of activity coefficients of oxygen as a function of aluminum and chromium contents in the melts. Clear positive deviations of the experimentally determined from the calculated activity coefficients of oxygen were found when aluminum was added to Ni-Co-Mo melts with or without chromium. From the results obtained in the range between 1 and 10 wt pct aluminum, the following equilibrium constants for the reaction 2 [Al] + 3 [O] ⇋ Al2O3 in the nickel based melts at 1600‡C were calculated: loga0 =-2/3 log [pct Al] - 3.94 for 0 pct Cr loga0 = -2/3 log [pct Al] - 4.21 for 10 pct Cr loga0 = -2/3 log [pct Al] - 4.81 for 20 pct Cr loga0 = -2/3 log [pct Al] - 5.06 for 30 pct Cr.