A new all-graphite transient heat treatment device has been developed for exposing captive powder samples to combustion-like thermal histories in inert gas environments, The device has been used to investigate the effects of short-time thermal treatment (2-118 s at peak temperatures of 700-2400 degrees C) on the oxygen reactivity of chars from a wide variety of organic precursors, with special emphasis on practical. solid fuels (coals, petroleum cokes, and biomass), The intrinsic reactivity of the chars is found to decrease by as much as 5 orders of magnitude between 700 and 2400 degrees C, even for hold times as short as 2 s. The higher reactivity materials generally show a higher propensity for annealing, leading to partial convergence in the data at heat treatment temperatures above 2000 degrees C. High-resolution transmission electron microscopy is used to provide some insight into annealing mechanisms. The implications of annealing for combustion and gasification processes are discussed.