Double refractory gold concentrates were subjected a two-stage microbial process to destroy both sulfides and carbonaceous matter. In the first stage, the well-known sulfide biooxidation process, which utilizes chemolithotrophic bacteria was used to oxidize sulfides and in the second stage carbonaceous matter is destroyed using the bacterium Streptomyces setonii. After biooxidation of sulfides in the first stage, cyanidation resulted in 81.1% gold extraction. The action of Streptomyces setonii in the second stage led to a reduction in the content of carbonaceous matter, which reflected positively in the preg-robbing and leaching behavior of the sample. Degradation of carbonaceous matter was affected by pulp density, temperature and retention time. After degradation of carbonaceous matter, gold extraction increased by 13.6% resulting in an overall extraction of 94.7%. The results depict a novel two-stage microbial process to degrade both sulfides and carbonaceous matter and increase gold recovery from double refractory ores. (C) 2004 Elsevier Ltd. All rights reserved.