In order to determine the activities of phosphorus and iron in liquid {Cu-Fe-P} alloys, the two coexisting phases of liquid {Cu-Fe-P} alloys + 〈Cu-Fe-P〉 solid solutions were brought into equilibrium with a mixture of Al2O3 + AlPO4 + FexAl 2O4 at temperatures of 1416K and 1526K. The oxygen partial pressures were measured with the aid of a solid-oxide galvanic cell of the type: (+)Mo / Mo + MoO2 / ZrO2(MgO) / {Cu-Fe-P} + 〈Cu-Fe-P〉 + 〈Al2O3〉 + 〈AlPO 4〉 + 〈FeAl2O4〉 / Fe(-) The equilibrium reactions underlying the experiments can be expressed by 2[P] cu + (5/2) (O2) + 〈Al2O3〉 = 2 〈AlPO4〉 and x[Fe]cu + (1/2) (O2) + 〈Al2O3〉 = 〈FexAl 2O4〉 The Henrian activity coefficient referred to 1 wt pct solution in pure liquid copper could be well expressed by the formula log fp o = (4.46±0.40) - (8.67±0.59)/(T/K). The iron activities referred to pure solid iron could be formulated as log a fe =- (0.37 ±0.12) + (500±200)/(T/K).