• There was a preferential sulphidisation on Cu(II) oxidised sites to form Cu(I)-S species. • Fe(III) oxidised sites remained unsulphidised at the Eh of 0.10 V vs. SHE. • The collector, O-isopropyl-N-ethyl thionocarbamate, did not reduce the ferric species. • The iron-contamination from oxidation of chalcopyrite had little effect on the flotation. • The high iron-contamination hindered the sulphidisation on Cu(II) oxidised sites. The controlled potential sulphidisation process, which has been widely applied and shown great potential as a remediation strategy for flotation of oxidised copper sulphides, still suffers from drawbacks such as low efficiency and difficulty to restore floatability of heavily oxidised sulphides. This study sought to understand the surface reactions of sulphidisation on oxidised chalcopyrite. The surface analysis by Cryogenic X-ray photoelectron spectroscopy revealed that the Cu(II) oxidised sites were sulphidised to form Cu(I)-S while the Fe(III) oxidised sites remained unsulphidised, at 0.10 V vs. standard hydrogen electrode, upon sulphidisation of the oxidised chalcopyrite. Analysis of the collector-treated chalcopyrite after sulphidisation showed that the collector, O-isopropyl-N-ethyl thionocarbamate, used in this study did not reduce the ferric species. Low iron contamination, emanating from oxidation of the chalcopyrite, had a negligible effect on chalcopyrite flotation, while high iron contamination adversely affected chalcopyrite flotation. The surface analysis further showed that high iron contamination not only enhanced the surface hydrophilicity of the mineral but also seemingly induced a steric hindrance to reaction of the sulphidising ions with the Cu(II) oxidised sites. [ABSTRACT FROM AUTHOR]