In larger combustion facilities, secondary reactions between char and chlorine and sulfur may be important. In this work, the interactions of chlorine and sulfur with biomass char during thermal conversion have been experimentally investigated. A laboratory-scale fixed-bed reactor was applied to study the capture of HCl and SO2 by biomass char in the temperature range of 400-950 degrees C. The observed reaction rate was sufficient for significant recapture of HCl and SO2 to occur under conditions typical for fixed-bed combustors. A maximum in the chlorine and sulfur retention existed at similar to 600 degrees C. Relatively high amounts of chlorine and sulfur could be retained in the char samples over the entire temperature range, compared to the inherent chlorine and sulfur content in the biomass. Spectroscopic and chemical analyses revealed that HCl was mainly captured by the inherent metal species, whereas SO2 was mainly captured by the organic matrix. As a result, the maximum chlorine retention was dictated by the inherent metal content of the biomass. Combustion of the chlorine- and sulfur-laden char samples resulted in high retention values of chlorine and sulfur in the ash at temperatures up to 600 and 800 degrees C, respectively. At higher combustion temperatures, chlorine and sulfur was gradually released to the gas phase, because of evaporation of KCl and dissociation of sulfates. A larger fixed-bed reactor was applied to simulate the combustion process that occurs in industrial-scale grate-fired boilers. Combustion of wheat straw samples in the large fixed-bed reactor confirmed that higher retention values of both chlorine and sulfur could be obtained, compared to the smaller laboratory reactor, presumably because of secondary capture.