The extent to which calcium carbonate deposition in an anaerobic reactor can be reduced by adding inhibitors (phosphate and iron) of calcium carbonate crystal growth was investigated. At several concentrations of the additive, the extent of precipitation was assessed in continuous experiments with laboratory-scale reactors, In the reactor, phosphate concentrations as low as 0.5-5 mg total-Pdm(-3) were found to severely inhibit CaCO3 precipitation. However, iron did not inhibit the deposition of CaCO3, which was found to be due to the fact that iron, in contrast to phosphate, only inhibits the growth of calcite and not the formation of aragonite, The results led to the conclusion that only additives which inhibit the formation of both aragonite and calcite can be used as effective inhibitors during anaerobic digestion. A chemical equilibrium model was developed and shown to be a useful tool to calculate the extent of calcium carbonate deposition during anaerobic digestion provided the proper apparent solubility product of calcium carbonate can be estimated.