The effect of oxygen adsorption on surface tension of liquid copper at two different temperatures (1390 and 1440 K) has been investigated. A combination of specially designed crucibles, He-Ne laser, and high-resolution photography in a highly controlled gas atmosphere was used for accurate measurements. Experimental data exhibits decreasing surface tension with increasing oxygen partial pressure. Discussion on the adsorption behavior of oxygen is presented based on thermodynamic models by assuming a hypothetical binary system of Cu-"Cu2O", in which "Cu2O" is considered as an associate molecule. Ideal adsorption model which assumes insignificant interactions between Cu and Cu2O on the surface exhibits the best fit to the experimental results. (C) 2014 Elsevier B.V. All rights reserved.