The oxygen concentration in molten binary Ni-based alloys was estimated by an electrochemical method using solid electrolytes. The electrochemical cell was Pt|Mo+MoO₂|ZrO₂(MgO)|O(in liquid)|Mo. The activity of oxygen determined from the electromotive force (EMF) of the cell increased when Cu was added to pure Ni. Namely, the oxygen concentration was higher than that for pure Ni only when the amount of Cu was between 1wt% and 30wt%. In binary Ni-Cr alloys, it was deduced that the addition of 15 and 20wt% Cr to pure Ni showed the lower oxygen concentration, although the liquid Ni-based alloys having 5, 7 and 10wt% Cr tended to have higher oxygen concentrations than pure Ni.