The effect of catalyst addition on thermal conversion characteristics and kinetics of waste biomass chars was investigated. The experiments were performed in a thermogravimetric analysis system, at non-isothermal heating conditions, under a carbon dioxide atmosphere. Reaction rates were determined by a power law model. The bulk of char gasification process occurred between 800 degrees C and 950 degrees C. Addition of alkali salts improved char conversion and increased the reactivity of the samples by lowering the reaction temperature. Peak values were reduced by 4 to 138 degrees C. The catalytic activity was enhanced when 10% Na2CO3, 10% CaCO3, and 10% Li2CO3 were used during the gasification of municipal solid wastes, sewage sludge, and waste paper, respectively. Alkali carbonates and especially binary mixtures of them increased the reactivity of the fuels by significantly reducing the activation energy. Activation energies ranged from 180 to 370 kJ/mol without catalysts, whereas from 82 to 353 kJ/mol with catalyst addition.