Coal-derived chars formed during air-blown gasification processes may rapidly lose reactivity, and this can limit the extent of their conversion. To study this effect, a laboratory-scale fluidized bed reactor has been modified to enable char samples to be prepared under strictly controlled conditions of temperature, pressure, particle size, gaseous environment, and residence time. This has been used to gain an insight into the deactivation of the chars as they form and during their subsequent residence time in the bed of the gasifier. The work shows that the char reactivity declines rapidly during its formation as part of the pyrolysis of the coal. This is thought to result from the rapid deposition of secondary, unreactive char within the pores of the material. In this work, it has been shown to occur within the initial 10 s in the reactor, but in reality, this effect probably occurred within 1 s. Temperature, pressure, and particle size have an impact on this process. Subsequently, and over a longer time scale, structural reorganization occurs in the solid char, and this anneals the structure by graphitization. This has been indicated by the identification of graphite, by X-ray diffraction in long-residence-time chars. The presence of steam in the fluidizing gas seemed to reduce the reactivity of small char particles, which could be due to the gasification of the more reactive parts. This effect does not seem to be present in data obtained with larger particles, and this is thought to be due to the greater influence of secondary char deposition in the larger particles. The reactivities of the largest particles formed over the longest time in this work are getting close to the reactivity values seen in char particles formed in a pilot-scale gasifier.