Performance and characterization of microorganisms in a laboratory-scale anaerobic baffled reactor (ABR) was studied. The ABR was fed continuously at organic loading rate (OLR) of 15.2 g COD/l.d with a substrate containing pre-tested combination of kitchen waste and activated sludge. The HRT (hydraulic retention time), C/N (carbon/nitrogen) and F/M (food/microorganisms) ratios were maintained at 3 d, 31.4 and 0.35 gCOD/gVSS.d, respectively. The amounts of fat, protein, cellulose, hemi-cellulose and lignin which are significant polymers for anaerobic digestion start-up were found to be in appropriate ranges. Results show reduction of pH in front compartments and its boost in successive compartments. This indicates proper placement of acidogens and methanogens in the ABR. Also, significant growth in height of ABR granule bed was observed in the system based on formation and retention of granules. Microorganism characterization reveals large population of Methanosarcina in front part of the reactor although toward the end the amount for Methanosaeta increased. While Methanobacterium, Methanosprilium, Methanococcus and Methanobrevibacter were observed in the ABR, higher percentage of Methanosarcina and Methanotrix indicates production of methane from acetate conversion. © (2013) Trans Tech Publications, Switzerland.