The rates of calcination of two types of limestones, ranging in particle size from 1 to 90 μm, were measured over the temperature range 516 to 1,000°C. A kinetic model based on the B.E.T. (Brunauer-Emmett-Teller method) surface area of the CaCO3 correlates the results over 5 orders of magnitude in reaction rate. The B.E.T. surface area of CaO formed by rapid calcination in dispersed-particle systems is 50 to 90 m2/g.