The attempt to explain the differences in sorption and flotation properties of auroferrous pyrites of several deposits from positions of inconstancy of iron to sulfur proportion (non-stoichiometry) has been undertaken. The effect of natural pyrites non-stoichiometric composition on their surface ability to oxidation, as well as, on the correlation between sulfite- and sulfate-ions in liquid phase during pyrite flotation has been shown. It has been found, that the collector chemisorption on pitytes with a non-stoichiometric composition has increased. The sulfur/iron ratio in natural auriferous pyrites correlates with their flotation properties at the range of pH 9-12.