Abstract A chemical equilibrium method has been used to determine the activity ofLa_2O_3 in CaF_2-base ESR slags and the effect of CaO, MgO and Al_2O_3 has beenstudied. It is shown that the activity of La_2O_3 (α_(La_2O_3)) increases rapidly with itsconcentration in the binary system La_2O_3-CaF_2 as represented by α_(La_2O_3)=N_(La_2O_3)~(1.59).Expressions of α_(La_2O_3) in slags containing CaO and MgO are: α_(La_2O_3(30%La_2O_3))=0. 0219+0. 0651N_(CaO) α_(La_2O_3(30%La_2O_3,10%CaO))=0. 0331+0. 0365N_(MgO)~(1.4)Thus, either CaO or MgO increases γ_(La_2O_3) and the effect of CaO is more mark-ed owing to its greater basicity. On the other hand, Al_2O_2 was found to lowerγ_(La_2O_3), especially when its content was in excess of 5%. Assuming that interactionbetween CaF_2, CaO and MgO when they coexisted would not alter the individualeffects upon α_(La_2O_3) as shown above, equations of more general applications maybe put forward for approximate calculations of α_(La_2O_3) in such ESR slags.
