The vapor pressure of phosphorus in molten carbon-saturated iron with phosphorus content of 0.20-0.92 mass% at 1473-1673 K was measured by double Knudsen cell mass spectrometry. In the molten carbon-saturated iron, phosphorus behaved according to Henry's law in the composition range. The activity coefficient of phosphorus in this conditions relative to infinite dilute solution, f(P), and the interaction parameter of carbon on phosphorus in molten iron, e(P)(C)', were determined to be as follows:
log f(P) = - T/300 + 0.82(+/- 0.05), e(P)(C)' = T/120 + 0.061(+/- 0.010).