Abstract The gasification of cellulose was examined from 350 to 650°C in helium and helium/water vapour mixtures up to total pressures of 2500 kPa. Particular attention was paid to the effects on the [ (CO 2 + H 2 ) CO ] molar ratio in the product gases of the selected additives—iron(III)oxide, zinc(II)chromite and potassium carbonate. The results are interpreted in terms of catalytic influences on key steps in the reaction sequence. Thus, the most effective additive in realising the highest gaseous fuel potential was potassium carbonate which may act as a catalyst for the carbon/steam reaction. Although the calorific value of the gases produced was not altered much by pressure, it did slightly affect the distribution of the product gases, probably by influencing the secondary reactions of tars. Scanning electron micrographs of the various solid samples are presented.