A sulphide-rich anaerobic sludge acclimated with a molasses wastewater was used to carry out studies on nitrate and nitrite reductions in continuously stirred batch reactors. It was shown that a COD/N-NOx ratio as high as 65.6 mg mg(-1) did not promote dissimilatory reduction of nitrogen oxides to ammonia. Denitrification was characterized by a probable accumulation of gaseous intermediates, nitric oxide (NO) and nitrous oxide (N2O), by sulphide consumption with concomitant elemental sulphur production and by an increase of the redox potential. In addition, sulphate reducers were completely inhibited by nitrogenous oxides. Cultures performed without any carbon source proved that denitrifiers were able to use sulphides as electron donors. Furthermore, while a lag phase preceded nitrate denitrification, nitrite was consumed immediately. Chemical reduction of nitrite by ferrous iron (Fe2+) was considered to be responsible for this difference. Evidence of such a chemodenitrification has been presented by using a sterilized sludge which kept its ability to reduce nitrite while it lost its capacity to use nitrate. Moreover, this chemical activity was favoured by Fe2+ addition. Finally, it has been suggested that during the cultures performed with non-sterilized sludge, a biological reduction of the ferric ions (Fe3+) would be coupled to nitrite chemodenitrification and would allow a regeneration of Fe2+.