The reaction rate equation for the flue gas desulphurization at low temperatures using fly ash/calcium sorbents has been determined in a fixed-bed reactor in the range of SO2 concentration, 2000 less than or equal to C less than or equal to 5500 (ppmv), temperature 52 less than or equal to T less than or equal to 67 degrees C and relative humidity 0.40 less than or equal to RH less than or equal to 0.90, working at conditions prevailing in the exhaust gases of coal-fired plants. The kinetic models discrimination procedure leads to a reaction rate expression based on the SO2 adsorption on a nonideal surface as the rate-controlling step, in terms of an exponential influence of the calcium conversion in the reaction sate. The influence of the SO2 concentration in the gas phase has been described by a partial-reaction order equal to 1. Concerning the temperature influence, the kinetic constant does not depend on this variable, which can be explained by a sorption equilibrium step previous to the reaction, the observed kinetic constant being an apparent parameter. The relative humidity has been concluded to be the most relevant variable, with a significant influence on the kinetic behaviour of the sorbent, which can be described by the fitting of the parameter which accounts for the nonideal solid surface to the relative humidity. From the obtained conclusions, an appropriate kinetic model based on three parameters has been proposed for the kinetic description of the desulphurization reaction under study, i.e. when a sorbent based on a fly ash-Ca(OH)(2) mixture 3/1 is used in the process.