A photoelectrochemical approach is utilized for studying the processes that take place on copper sulfide mineral surfaces in alkaline solutions, either in absence or presence of potassium ethyl - xanthate. It is found that photoresponses are closely related to the mineral stoichiometric composition which also determines its semiconducting or metallic character. It appears that in the potential interval where xanthate anions can be adsorbed, surface composition and reactivity are closely linked