Experiments conducted to determine the reactivity of crushed limestone in acidic water were designed primarily to simulate the neutralization of acidic streams by barriers of crushed limestone, although the results obtained appear to be more widely applicable. Acidified water, at pH values ranging from 2 to 7, was circulated over crushed limestone in the size range 2-1/2 to 4 inches. Water samples were withdrawn periodically for determination of alkalinity and acidity, from which the kinetics of the two slower chemical reactions that limit the overall process could be determined. These are the reaction between hydrogen ions and solid calcium carbonate, and the exsolution of carbon dioxide. For each of these reactions the dependence of the rate constant on temperature, ionic strength, and water turbulence was established. Two linked differential equations were derived, and solved simultaneously to obtain a description of the neutralization process.