The effects of temperature and steam concentration on Spheron-6 carbon black with and without impregnated K&O, and diluted with alumina have been measured by in-situ electron spin resonance (esr.). For samples in flowing helium, heating from 290 to 1070 K results in a small increase in the carbon black resonance linewidth but a large and mostly irreversible increase in the resonance linewidth of carbon black with K&O, at >650 K. The effect of steam concentration on the e.s.r. spectra of the carbon black at 875 K has been studied; an effect is observed only for carbon black with K,CO,. Increasing the steam concentration from 0 to 6.5 vol'% results in reversible narrowing of the linewidth while hardly changing the free radical concentration. The mechanism of the line-broadening of the resonance resulting from heating the K,CO,-impregnated carbon black may be due to one of two processes: an unresolved nuclear hyperfine interaction between the unpaired electron of thecarbon free radical and potassium, or an increased mobility of charge carriers produced by the potassium. The reversible effects of steam are attributed to a modificartion of these processes and may be related to the mechanism of catalysed gasification of coal.