A study has been conducted on the effect of an HZSM-5 zeolite catalyst used in situ in a conical spouted bed reactor in the flash pyrolysis of sawdust at 400 degrees C. The presence of the catalyst allows great changes in the yields of gas, liquid, and char. Thus, the yield of gas increases as the catalyst amount is increased, whereas the yield of liquid decreases significantly and the yield of char decreases slightly. The values of the kinetic constants corresponding to the kinetic scheme proposed for the catalytic pyrolysis have been calculated. This scheme includes two new steps that are different from those of thermal pyrolysis, namely, the transformation of the liquid into gas and char. As a consequence of the use of the catalyst, the composition of the bio-oil undergoes a drastic change, given that the heavy organic fraction is partially transformed to the aqueous fraction due cracking, whereas the aqueous liquid fraction undergoes reactions of dehydration, decarboxylation, and decarbonylation. These reactions explain the increase in the generation of CO2, and particularly CO, when the amount of catalyst is increased. A detailed study is performed on the effect of the amount of catalyst on the composition of the bio-oil.