A quantitative study of the influence of inlet sulfur dioxide concentration (600-3000 ppm), relative humidity (20-60%), reactor temperature (56-86 degrees C) and different amounts (0-30 wt.%) of inorganic additives (NaCl, CaCl2 and NaOH) on gas desulfurization has been carried out in a continuous downflow fixed-bed reactor containing calcium hydroxide diluted with silica sand. Results show that the reaction rate does not depend on sulfur dioxide partial pressure (zero-order kinetics) and that the temperature and the relative humidity have a positive influence on reaction rate. An apparent activation energy of 32 kJ/mol Ca(OH)(2) has been estimated for the reaction. An empirical reaction rate equation at 71.5 degrees C and 36.7% relative humidity that includes the type and amount of additive is proposed. It has been found that calcium chloride is the best additive studied because it allows for a higher degree of sulfur dioxide removal.