<p>Phase equilibria of the Cu-Fe-Sn-O-SiO₂ system at iron and copper saturation as well as at Cu-Fe-SiO₂ triple saturation were measured at 1200–1300°C using the well-established equilibration-quenching method in closed systems. The phase analyses were carried out from polished sections using electron probe microanalysis and laser ablation–inductively coupled plasma–mass spectrometry. The solubilities of copper and tin in the molten silicate slag were measured as well as the compositions of the solid iron alloys and the molten copper–tin–iron alloy. Additionally, the experimental conditions also allowed the determining of the solubilities of tin and copper in solid fayalite (Fe₂SiO₄) at 1200°C. The results enable reliable modeling of the thermodynamic properties of iron silicate slags at iron saturation, the solid iron solutions containing copper and tin, and the tie-lines between solid iron and liquid copper–tin alloys.</p>