Members of the genus Clostridia can reduce Pd(II) ions to form metallic Pd nanoparticles (bio-Pd). Cultures of C. pasteurianum BC1 were used to generate bio-Pd, which is primarily formed on the microbial cell wall. Batch experiments using C. pasteurianum BC1 cells loaded with bio-Pd showed efficient reduction of the organic azo dyes, methyl orange and Evans blue, while little reduction of dyes was observed in control experiments using Pd-containing heat-killed microbial cultures or Pd-free viable bacterial cultures. Degradation of azo dyes was found to occur via reductive hydrogenation of the azo-linkage. Molecular hydrogen, which is concomitantly generated by C. pasteurianum, is used in the reduction reaction. The process described in this study is a potentially viable alternative to current groundwater and wastewater treatment technologies that fail to adequately degrade the large quantities of hazardous spent textile dyes that are discharged into the environment each year. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.053301jes] All rights reserved.