In this work, the forms of potassium and chlorine from oxy-fuel co-combustion of lignite coal and corn stover under atmospheric pressure were investigated. In order to check transitional stage, the feedstocks were combusted stepwise, first by pyrolysis to form coke under N2 environment and later by coke combustion into the ash at 850 °C in O2/CO2 atmosphere. The results show that an increase in blend ratio from 15% to 40% results in an increase in water-soluble potassium in the feedstock and the ashes from 0.15% to 0.4% and 0.015% to 0.038 % in weight respectively. The water-soluble potassium is present mainly as KCl and K2SO4. For ammonium acetate soluble potassium, a similar trend to water-soluble potassium is presented but with a much lower content of potassium. The bound potassium in the fuel matrix exists, likely in the form of AlKSi2O6. Chlorides are present mainly in the form of KCl which is the dominant water-soluble compound.