The contribution of extracellular metabolites (EM) to the decolorant activity of newly isolated photosynthetic bacteria was observed. The decolorization process was considered to occur by two paths: photochemical decolorization by EM and photobiological decolorization by photosynthetic bacteria. In addition, the decolorization of several azo dyes by EM under black light and fluorescent light irradiation was investigated. It was found that EM were capable of decolorizing the azo dyes directly under visible light irradiation, and the overall dye decolorization followed first-order decay kinetics. Moreover, the decolorization reaction is a nonenzymatic reaction and the unknown metabolite with the decolorizing ability had an apparent molecular weight lower than 3 kDa as determined by ultrafiltration. In addition, its decolorization activity was stable even after heating sterilization at 121 degrees C for 10 min. Furthermore, the decolorization rate increased with increasing optical intensity, temperature and EM concentration, and decreased with increasing initial dye concentration. Decolorization of dye was best at pH 8.