The thermodynamic data on interaction parameters of various elements in iron are regarded as an essential basis of steelmaking technology. Several experimental techniques for determining the interaction parameters between elements in molten iron have been developed so far. This article introduces the evaporation technique and the chemical equilibrium technique out of them. The former typified by a) Knudsen-mass spectrometry and b) Gas transpiration method is a straightforward way to obtain the interaction parameters because the activity of elements bears a proportional relationship to their vapor pressure. This technique is effective against the elements with high vapor pressure in molten iron. On the other hand, the latter is often carried out under the equilibrium state of the reaction that simulates the chemial reaction in refining process etc. to acquire practical values for steelmaking. c) Distribution coefficient method, d) Equilibrium measurement of chemical reaction, and e) Equilibrium measurement of oxidation via Ag are explained as a chemical equilibrium technique. The theory and characteristics of each method are described in this article.