The effect of oxygen on the surface tension of liquid Ag-Sn alloys used as one of the main components in lead-free soldering alloys was measured by the sessile drop method. The surface tension of liquid Ag-Sn alloys decreased with increasing tin content. The effect of oxygen was investigated for oxygen partial pressure ranging from about 10^-17 to 10^-12 atm. The effect of oxygen on the surface tension of liquid Ag-Sn alloys is closer to pure tin rather than pure silver. Thermodynamic simulation using Butler's equation showed that the surface of liquid Ag-Sn alloys is enriched with tin. Accordingly, the effect of oxygen adsorption on the surface tension of liquid Ag-Sn alloys was considered to be closer to that of tin than silver.