Most of the gold and silver produced worldwide are extracted by the cyanidation process. The recovery of the precious metals involves two distinct operations: the oxidative dissolution of gold and silver by an alkaline cyanide solution, and the reductive precipitation of metals from the solution. From the cyanidation point of view, gold and silver ores can be classified as free milling, and refractory ores. The term "refractory ore" defines those materials that when submitted to a conventional cyanidation process, show low recoveries (< 80%) or high consumption of reactants [Weir, D., Berezowsky, M., 1984. Gold Extraction from Refractory Concentrates, Sherrit Research Centre, Alberta, Canada, pp. 1-26; Haque, K.E., 1992. The Role of Oxygen in Cyanide Leaching of Gold Ore, CIM Bulletin 85963, pp. 31-38]. These refractory ores are usually pretreated by some oxidizing process after which gold and silver can be recovered by standard cyanidation process. Since ozone gas (O-3) is a strong oxidizing, it may be regarded as a promising alternative in the treatment of refractory ores. The present work summarizes the results obtained when two pyritic refractory ores from Mexican sites (samples A and B), were pretreated with an oxygen/ozone stream in acid media before cyanidation. Two contacting methods were studied: the indirect method (contacting the ore three times with ozone saturated water), and the direct method (direct addition of ozone to the mineral slurry). Sample A did not show any difference in recoveries with indirect pretreatment, while the direct pretreatment reduced the cyanidation time for maximal gold and silver recovery from 40 to 24 h. Sample B, only tested with indirect contact method, increased the gold recovery from 53% to 88% and the silver recovery from 26% to 78%. (c) 2005 Elsevier Ltd. All rights reserved.