The pyrolysis of biomass was conducted using a microwave heating system operated at 2.45 GHz. Additives, such as silica carbon, activated carbon, coke produced through microwave-assisted pyrolysis, potassium carbonate, and sodium hydrate, were investigated for their effects on the sawdust pyrolysis. It was found that additives had significant effects on the yields and properties of the final products. By employing silica carbon, the gas and liquid yields were relatively low, due to the non-uniform mixing caused by big density difference between the sawdust and silica carbon particles. However, with potassium carbonate and sodium hydrate, the pyrolysis products are mainly gases, since both additives can absorb microwave strongly, making extremely high temperature inside the sawdust. The liquid products were analyzed by electrospray FT-ICR MS. Potassium carbonate can simplify the bio-oil composition and reduce compound types, which is beneficial to its further applications. The pyrolysis is highly influenced by heating rate and a first order was found for the reaction. Additives under microwave irradiation would enhance the heating rate, while they mainly act as catalysts instead of enhanced heating. (C) 2014 Elsevier B.V. All rights reserved.