<p>Metallurgical slags often contain transition metal oxides such as iron and chromium oxides. These oxide components can exist in multiple valence states and are often considered the end members of solutions. This study investigates the unusual thermodynamic behavior of transition metal oxides in slags, where the activity of the oxide component remains greater than zero, even when its concentration is zero. This phenomenon cannot be described using conventional solution models that treat these oxides as end members. Therefore, considering the FeO–FeO_1.5 binary system as an example, a novel formalism is proposed to describe the thermodynamic behavior of transition metal oxides by modifying the entropy of mixing in the conventional regular solution model. This new approach enables better prediction of slag properties for metallurgical processes.</p>