A statistical approach has been adopted to evaluate the Pb-Zn selectivity of various organic collectors of the mercaptobenzothiazole (MBT) and aminothiophenol (ATP) types, in the flotation of lead and zinc minerals. Six replicated tests were performed using each collector in order to obtain an estimate of a statistical population characterized by an average and a variance. Comparison of these statistical populations indicated the most selective collectors. The selectivity exhibited by the collectors was then related to their molecular structure.