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The adsorption of phosphate onto calcite was studied in a series of batch experiments. To avoid the precipitation of phosphate-containing minerals the experiments were conducted using a short reaction time (3h) and low concentrations of phosphate (⩽50μM). Sorption of phosphate on calcite was studied in 11 different calcite-equilibrated solutions that varied in pH, [Formula: see text] , ionic strength and activity of Ca²⁺, CO₃^2- and HCO₃^-. Our results show strong sorption of phosphate onto calcite. The kinetics of phosphate sorption onto calcite are fast; adsorption is complete within 2–3h while desorption is complete in less than 0.5h. The reversibility of the sorption process indicates that phosphate is not incorporated into the calcite crystal lattice under our experimental conditions. Precipitation of phosphate-containing phases does not seem to take place in systems with ⩽50μM total phosphate, in spite of a high …