The present study was undertaken to clarify the purpose of using reactive gases during the thermochemical processing of biomass. Conversions observed during pyrolysis (He) have been compared directly with gasification in H-2, CO2 and steam-helium mixtures. Experiments have been carried out at pressures between 1 and 20 bar, in a fixed-bed ＇hot-rod＇ reactor, operating at a relatively low heating rate (10 degrees C s(-1)). Conversions in helium were between 85 and 88%, leaving little scope for improvement by the use of reactive gases. Up to 95% has been achieved elsewhere by heating at faster rates. As the use of steam or hydrogen also tends to enhance heat transfer to particles, the use of reactive gases may still be justified when particles are heated rapidly to lower reactor temperatures, and/or when residence times at peak temperature are very shea. In pilot or industrial scale fluidised-bed operation, loss of fuel through elutriation of fines is usually suppressed by a combination of increased fuel particle size and the use of reactive gases. The present study shows that comparable or higher conversions can be achieved by operating above 800 degrees C in an inert gas, provided elutriation is suppressed by design options other than increasing particle size, which tends to produce unreactive chars. The use of reactive gases such as steam would appear to partially compensate for possibly non-optimal choice of fluidising parameters.