The adsorption of butyl ethoxycarbonyl thiourea (BECTU),I a commercially used collector for copper sulfides and precious metals, on chalcopyrite was studied as a function of pH in the presence of oxygen and nitrogen. The adsorption of BECTU was followed in solution by using UV-visible spectroscopy and on the chalcopyrite surface by using X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), zeta potential and flotation measurements. The adsorption of BECTU increased in the presence of oxygen or with decreasing pH, behaviour which was reflected in the flotation recovery of chalcopyrite. SIMS analysis provided strong evidence for the presence of a 1:I Cu-BECTU surface complex. XPS showed that the adsorption of BECTU removes surface oxidation products. Zeta potential results indicated the presence of an adsorbed, positively charged BECTU species, the concentration of which increases with decreasing pH values. On the basis of these results, a mechanism of adsorption of BECTU on chalcopyrite involving surface complexation is proposed.