Routine monitoring of landfill leachates has been extended to include characterization of suspended solids recovered by filtration. Calcite is consistently identified as a suspended solid, with less frequent reports of quartz and clays (kaolinite, illite, chlorite). Morphologically, calcite occurs as discrete grains, coatings on quartz sand and as microconcretions. Preliminary stable isotope data for seven samples generally show positive δ 13C values (relative to PDB) up to +3.5‰ and δ 18O values between −5 and −8‰, consistent with an origin through precipitation from leachate. Geochemical modelling of leachate compositions for the same samples indicates that the leachates are saturated with respect to calcite, and that the degree of supersaturation decreases for older samples. Mass balance considerations show that the proportions of methane and carbon dioxide observed for landfill gas do not reflect the amount of bicarbonate that is potentially available from the anaerobic decomposition of putrescible waste. Overall, putrescible waste has the potential to form a maximum of 1.9 g of calcite for every gram of waste, although values less than this are likely to be achieved in practice. From these differing lines of evidence, there can be no doubt that calcite precipitation should be expected to take place within landfill systems as an essential part of the waste degradation and stabilization process, and should be considered in modelling both gas evolution and carbon emissions.