Sorbents to be used in flue gas desulfurization processes at low temperature were prepared from fly ash and calcium hydroxide in the presence of calcium sulfate at different initial ratios, hydration times and temperatures. The aim was to study the controlling mechanism and to find the kinetic model for the hydration reaction, in order to optimize the sorbent preparation process. The conversion of Ca(OH)(2) with the hydration time was determined for all the samples. The shrinking core model for fluid-solid reactions was used. The chemical, diffusion and mixed control models were tried, showing the results a different controlling mechanism depending on the preparation conditions used. Because the chemical control model fits experimental data with standard deviations less than 10% in all the range of variables studied, the influence of the preparation variables was included in this model. Activation energy of 57.7 kJ/mol was calculated together with the parameters in the mathematical equation that allows predicting the optimal preparation conditions of a desulfurant sorbent. (C) 2004 Elsevier Ltd. All rights reserved.