The natural melanoidin from alcohol distillery biogester effluent and synthetic melanoidin, which was prepared in the laboratory from equimolar amounts of glucose and glycine were decolorized by Bacillus subtilis in shake flash culture. After eight days of fermentation at an initial bacterial level of 2.9 x 10 E 9 cfu/mL, natural melanoidin was 61.1 percent decolorized as measured by absorbance readings at 475 nm: the corresponding decolorization of 0.041 M synthetic melanoidin was 71.9 percent. Gel chromatography through Sephadex G-100 of natural melanoidin before and after microbial action showed molecular weight (MW) values of 35.6 and 33.5 kD, respectively; the corresponding MW values of freshly prepared and decolorized synthetic melanoidin were 42.6 and 37.7 kD, respectively. Values of the instrinsic viscosity, which is proportional to MW, of natural melanoidin before and after decolorization were 18.46 and 16.96 mL/g, respectively; corresponding viscosity values for synthetic melanoidin were 19.77 and 17.16 mL/g, respectively. After bacterial action both natural and synthetic melanoidins showed greater absorbance at 220-350 nm and lower absorbance at greater than 370 nm but showed reduced infrared intensities corresponding to C=O, C=C and O-H stretching vibrations; however the C-O infrared intensity increased. The results suggest partial depolymerization and oxidative degradation of both melanoidins caused by the microorganisms