This paper presents some important findings on the passivation of pyrite and pyrrhotite minerals. Polyethylene polyamines, such as triethylenetetramine (TETA), diethylenetriamine (DETA), used alone in dilute solutions or combined with potassium amyl xanthate (p.a.x.) can provide an effective protective layer for sulfidic ores and significantly reduce the penetration of atmospheric oxygen and the oxidation of these ores. Experimental studies were carried out systematically on both small particles and coarse samples using both diluted hydrogen peroxide solutions and atmospheric oxygen as oxidants. In all cases, a remarkable diminution of oxidation of the coated samples was observed. The study also demonstrates that solutions of polyethylene polyamines such as DETA and TETA are relatively strong reducing agents while showing good buffering capacities as well because of their nucleophilic functional groups. These properties give them additional advantages over other coating agents. The presence of these compounds also prevents the proliferation of the bacterium Acidithiobacillus ferrooxidans that plays an important catalyzing role in the oxidation of sulfides and acid mine generation. Polyethylene polyamines are effective coating agents to prevent surface oxidation of sulfidic minerals. (c) 2005 Published by Elsevier Ltd.