The composition of ash deposits formed in biomass combustion systems can be significantly modified by the sulfation of alkali salts. In order to assess the efficiency of this process, the conversion of alkali chlorides into alkali sulfates occurring in deposits at surface temperatures representative of those in biomass boilers (350-650 degrees C) was quantified by means of specific short-term experiments with realistic deposits. The influence of operating conditions (reaction time, surface temperature and sulfur concentration) was characterized and the results were used to derive a kinetic expression for the in-deposit sulfation of KCl. This kinetics was compared with those proposed by other authors for solid NaCl and molten KCl. Contrary to previous results for solid NaCl, but in agreement with observations for solid KCl and calculations for molten KCl, the proposed kinetics predicts a relevant effect of sulfation on the properties of deposits formed in biomass-firing boilers even in the short-term scale. (C) 2015 Elsevier B.V. All rights reserved.