In the batch reactor system, denitrification was almost complete while the C/N ratio of each of the four different carbon sources (methanol, acetic acid, glucose, benzoic acid) was properly controlled. The minimum C/N ratio required for nearly complete denitrification (92-97%) using an aromatic carbon source (benzoic acid) was significantly higher than that using the other three non-aromatic sources. For non-aromatic carbon sources, the minimum C/N ratio required for nearly complete denitrification (97-100%) increased with an increase of molecular weight. After nearly complete denitrification, the carbon residual could be further reduced to methane gas. Under denitrifying loadings of 0.07-0.10 g-N/g-VSS/day, the endogenous denitrification efficiencies obtained were 9.2-17.8%. If insufficient carbon was supplied for denitrification, accumulation of intermediate nitrite species could be observed (glucose gave the highest and benzoic acid was the least). If methanol or benzoic acid as a carbon source was over-supplied, carbon breakthrough would occur.