The effect of the following catalysts: MS (Molecular sieve) 4A, Fe2O3/MS 4A, CoO/MS 4A, NiO/MS 4A, MgO/MS 4A, PtO/MS 4A, Al2O3/MS 4A, La2O3/MS 4A, Cl-/MS 3A, SO42-/MS 3A, Na2O/MS 3A, CaO/MS 3A, K2O/MS 3A, CoO/ZrO2, NiO/ZrO2, La2O3/ZrO2, NiO/CaO-ZrO2, Cl-/ZrO2, SO42-/ZrO2, Na2O/ZrO2, CaO/ZrO2, and MgO/ZrO2, on chemical profile of the products from microwave-assisted pyrolysis of biomass was studied. A microwave oven with a frequency of about 2.4 gigahertz, and a power of about 1-1.3 kilowatt was used to pyrolyze aspen (Populus tremuloides). The steam that evolved was removed from the oven and passed to a catalyst column where the temperature was controlled at about 350-600 degrees C, and the converted vapors were then condensed to bio-oils. The chemical profiles of the bio-oils were determined using gas chromatography-mass spectrometry. Solid acids were proved to be effective catalysts to decompose pyrolysis vapors, while solid alkaline and other catalysts do not seem to affect the composition of the liquid products from microwave-assisted pyrolysis. Increasing the temperature of the catalyst bed and the ratio of catalysts to biomass adversely affected the liquid yield.