The aim of this study is to understand the homogeneous vapor-phase reactions in lignocellulosic biomass pyrolysis by using two separated reactors combined with an advanced analytical technique. The fast pyrolysis of elm was conducted in a microfluidized-bed reactor (MFBR) at 500 degrees C. The formed pyrolysis tar would be immediately introduced into a secondary tubular reactor (STR), in which the secondary reactions occurred and their product components were monitored by a photoionization mass spectrometry in real time. The mass yields of solids, condensables, and gases with different secondary reactions were measured. Meanwhile, the aerosol particles (>2.0 mu m) were collected and their components that can be dissolved in methanol were characterized by GC-MS/FID. The results show that secondary reactions became active only when the temperature was higher than 500 degrees C. Temperature and residence time (RT) have a combined effect on secondary reactions (e.g., mass balance, tar composition, etc.) and the formation of polycyclic aromatic hydrocarbons (PAHs). The temperature is critical to the secondary reactions, in which, however, the long RT would contribute to decreasing the required temperature and to the formation of large PAHs. Furthermore, the possible routes for PAHs formation were proposed and the reaction conditions that impact them are also discussed.