In this work we investigated the effects of K2CO3 (1, 100, 1000 and 10,000 mg kg(-1)), KOH (1000 mg kg(-1)) and KCl (1000 mg kg(-1)) on the primary products of cellulose fast pyrolysis in vacuum (5 mbar) and at atmospheric pressure (1000 mbar) using a dedicated screen-heater set-up. The screen-heater combines fast heating of the cellulose sample with very fast (milliseconds) removal (high escape rate) and quenching of the reaction products. The solid residue, condensed product, non-condensable gas, levoglucosan and glucose yields obtained from the screen-heater experiments are compared with the product yields from the fluidized bed reactor to elucidate potassium catalysed reactions in the hot vapour phase. Potassium was found to be catalytically active even when the escape rate (milliseconds) of the product was extremely high. Consequently, significantly lower condensed product and sugar yields were obtained. The production of non-condensable gas could almost completely be ascribed to thermal reactions in the vapour phase when the potassium content is low but was predominately produced at the hot reacting particle when the potassium content is high. It was found that in the case of levoglucosan the anions were active in potassium the following order Cl- > OH- > CO32-. The activity in reducing the hydrolysable anhydro-sugars (levoglucosan and oligomers), expressed as glucose recovery, shows the following order, OH- > Cl- > CO32-. Vacuum could help to improve the oil and sugar yield by fast removal of the products from the hot reacting sample. However, this only holds for cellulose with a low potassium (<100 mg kg(-1)) content. (C) 2017 Published by Elsevier B.V.