We report on an extensive experimental study of char thermal deactivation based on reactivity experiments of different fuel chars collected from different reactors. The reactors include a Thermogravimetric Analyser (TGA), an entrained flow reactor and utility boilers. The results show that different fuels have different deactivation propensities. In the course of heat treatment in the TGA, some of the fuels present a gradual formation of two fractions of different reactivity. The mass-based reactivity of the least reactive phase approaches that of a commercial graphite at high heat treatment temperatures. Likely mechanisms responsible for the observed deactivation of the chars studied are discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.