•Potassium influence was characterized during fast pyrolysis of lignocellulosic biomass.•Potassium lowered thermal decomposition temperature of biomass.•Char formation as well as its agglomeration were also promoted by potassium.•Potassium facilitated chemical reactions, such as dehydration and demethoxylation.•Aromatic compounds were identified from non-condensable gas fraction. TGA results indicated that the maximum decomposition temperature of the biomass decreased from 373.9 to 359.0°C with increasing potassium concentration. For fast pyrolysis, char yield of potassium impregnated biomass doubled regardless of pyrolysis temperature compared to demineralized one. The presence of potassium also affected bio-oil properties. Water content increased from 14.4 to 19.7wt% and viscosity decreased from 34 to 16.2cSt, but the pH value of the bio-oil remained stable. Gas chromatography/mass spectroscopy (GC/MS) analysis revealed that potassium promoted thermochemical reactions, thus causing a decrease of levoglucosan and an increase of small molecules and lignin-derived phenols in bio-oil. Additionally, various forms of aromatic hydrocarbons, probably derived from lignins, were detected in non-condensed pyrolytic gas fractions.