The effect of mineral oxidation on the flotation performance of chalcopyrite was investigated at pH 9.5 as a function of pm1icle size and collector concentration. The contact angle of the chalcopyrite pm1icles was measured to inform about their surface hydrophobicity while X-ray photoelectron spectroscopy (XPS) was used to identify the species present on the surface responsible for chalcopyrite flotation. It was found that the collectorless flotation of chalcopyrite was relatively high but decreased after conditioning with hydrogen peroxide. Addition of xanthate collector improved chalcopyrite flotation but could not restore it to the level observed in the absence of hydrogen peroxide, especially for the com·se and fine pm1icles. XPS analysis revealed that oxidation of chalcopyrite occUlTed after hydrogen peroxide addition with formation of surface iron oxide/hydroxide and sulphate species, which explains the decrease in particle hydrophobicity and flotation performance. Refereed/Peer-reviewed