An optically accessible pyrolysis reactor was developed to analyze the in-situ reaction of biomass feedstock during the initial stage of fast pyrolysis. A Fourier Transform Infrared (FTIR) Spectroscopy, analytical instrument was modified to allow the light source to pass through the optical pyrolysis reactor for time-resolved measurements during the pyrolysis reaction. Cellulose, red oak, corn stove organosolv lignin, and a mixture of solid heat carrier with organosolv lignin were tested with the reactor. Evolution trends of the gas products and functional groups within the products were investigated as a function of time with the FTIR instrument. Residence times were regulated using a mass flow controller to allow different flow rates of sweep gas entering the reactor. Details of the reactor, including automated sample injection, temperature profile, and feedstock measurements are documented and discussed. The data collected in the experiments are shown to be useful for temporal analysis of pyrolysis products as a function of the type of feedstock. The analytical technique is proven to be repeatable and capable of capturing temporally resolved information that is otherwise difficult to obtain in conventional pyrolysis reactors. This can potentially provide improvement in understanding the kinetics of biomass fast pyrolysis.