In this study, the research was designed to answer two questions: how does the degree of surface oxidation affect coal flotation and how to improve the flotation of coals with different degrees of surface oxidation? The degree of coal surface oxidation was quantified as the percentage of oxidized carbon by X-ray photoelectron spectroscopy (XPS), and flotation tests were conducted to determine the flotation behavior of coals with different degrees of surface oxidation. It was found that coal flotation performance decreased with an increase in the degree of surface oxidation. A critical degree of coal surface oxidation was observed in flotation and, above it, the true flotation of coals was impossible. To improve the flotation of oxidized coals, diesel (a nonpolar collector) and Triton X-100 (a polar collector) were mixed as a composite collector to enhance the hydrophobicity of both un-oxidized and oxidized coal surfaces simultaneously. Flotation tests demonstrated a significant increase in the flotation of oxidized coals after the addition of X-100. A close relationship between the degree of coal surface oxidation and the proportion of X-100 in the composite collector used to achieve the optimal coal flotation was identified: the higher the degree of coal surface oxidation, the higher the proportion of X-100 required. This study indicates that the best composite collector used to float an oxidized coal may be formulated based on the degree of surface oxidation. (C) 2017 Elsevier B.V. All rights reserved.