Thermogravimetric analysis (heating rates between 5 and 15 K/min up to 873 K) has been carried out in air of chars, generated from the pyrolysis of wood and wood impregnated with (NH4)(2)HPO4 and (NH4)(2)SO4 (5%, dry wood basis) which are widely used as flame retardants. Salts significantly alter the char reactivity, but a mechanism consisting of three parallel reactions always provides a good description of the measurements. The estimated activation energy and reaction order of the most important combustion reaction, which causes the release of about 71-80% of the volatile matter, are nearly the same for untreated and (NH4)(2)SO4 impregnated samples (about 190-200 kJ/mol and 0.86). A lower activation energy (about 150 kJ/mol) and a higher reaction order (1.35) are obtained for (NH4)(2)HPO4 treatments. Scanning electronic microscope (SEM) images show significant modifications in the morphological structure of both wood and char as a consequence of salt impregnation. (c) 2007 Elsevier B.V. All rights reserved.