In this paper, the effect of secondary reactions in vapor phase on the yield and composition of pyrolytic lignin (PL) oligomers was investigated. The Pine wood material was pyrolyzed at 500 °C in a fluidized bed reactor, and the vapors were transferred to a downstream tubular reactor operated at residence times between 0 and 15 s and temperatures of 400, 500, and 550 °C. Relatively small changes in yield and composition of PL oligomers were observed when the tubular reactor was operated at 400 °C. However, as the temperature in the tubular reactor increased (500 and 550 °C), the effect of residence time became more pronounced and the rate of thermal degradation of pyrolytic lignin increased. The pyrolytic lignin fraction was isolated from the bio-oil and analyzed by UV-fluorescence, TGA, Py-GC/MS, FTIR, ESI-MS, and 1H NMR. The main change observed in the composition of PL oligomers (by Py-GC/MS and 1H NMR) as a function of the vapor phase reaction time and temperature was the removal of methoxyl groups. FTIR and 1H NMR analyses indicated the formation of C═O groups. The change in molecular weight distribution observed by ESI-MS suggests that both cracking and addition reactions compete during secondary vapor phase reactions in pyrolysis reactors.