Chemical changes in anoxic tidal waters, emerging from the mangrove forest of Cananeia, (48° W, 25° S; Brazil), were interpreted using a stoichiometric approach. Significant depletion of magnesium and sulphates were observed and were apparently due to magnesium-iron replacement in octahedral sheets of clay minerals, and sulphate reduction, respectively. Iron release from clays, after bacterial-mediated reduction, apparently reacted with sulphides produced by sulphate reduction to form nono- and di-sulphides. Production of 41.5% of FeS and 58.5% of FeS2 satisfactorily explained all the stoichiometric relations between concentration changes of magnesium, sulphates, sulphides, iron and alkalinity expected by the model. As a consequence, the decomposition rate of organic matter in these prevailing anoxic conditions, would mainly stem from sulphate reduction (90% against 10% for ferrireduction). The simultaneous bacterial processes of uptake and release of ammonium included in the model would explain why the mangrove forest does not export significant amounts of nitrogen toward the adjacent lagoon.