As a fundamental study on the physico-chemical properties of slags containing fluoride ion, a measurement has been made on the activity at the equilibrium of hydrogenwater vapor gas mixtures with the slags of Fe_tO-CaO-CaF₂ ternary system contained in solid iron crucible at 1400℃. From the results obtained, the activity of iron oxide and the ratio of ferrous to ferric iron (Λ=Fe^(3+)/Fe^(2+)) have been determined as a function of slag composition. The addition of F- anions increased the activity coefficient of iron oxide exponentially, but it seemed to be independent on Λ-values. The obtained results are as follows, lnγ_(Fe_tO)/lnγ^0_(Fe_tO) =1.78 N_F-