Second-order orthogonal experimental design has been used to examine the effect of mechanical activation, mass sample, heating rate and molar fraction of carbon dioxide in the environmental atmosphere on the thermal decomposition of calcite. The mathematical model of the activation energy (E(alpha)) for various mechanisms was examined by the Coats-Redfern equation using the best-fit procedure and the method of the shape of the TG/DTG curves. The best-fit procedure gave the second-order reaction mechanism (F2) as the predominant mechanism while the shape method gave the contacting area geometrical mechanism (R2) and the two-dimensional diffusion mechanism (D2) as the predominant mechanisms. The mathematical models for the change of enthalpy (Delta H) and the temperature of the maximum rate of decomposition (T-max) were found, but unfortunately only the T-max model was of significance.