A bench scale pyrolysis reactor was developed to study the evolutions of species during the pyrolysis of biomass and for different product residence times. The product residence times were controlled by the flow rate of sweep gas inside of the reactor. Two product residence times of 0.9 and 1.4 seconds were studied. The reactor has optical access which allows for in-situ diagnosis of the products in the reactor. In this study, Fourier Transform Infrared Spectroscopy (FTIR) was adapted to allow for the I.R. beam to be drawn out of the standard sample chamber for propagation through the pyrolysis reactor and subsequent spectral analysis. The in-situ FTIR spectra allowed for time-resolved studies of the product evolution during pyrolysis. The two biomass substances that were studied were cellulose and lignin. Their evolution trends were studied and reported as well as the approximate evolution times for certain species classified by bond groups. The data reported shows that the methods established in this work can be used for temperal analysis of pyrolysis for various feedstocks and test conditions, potentially helping to improve the understanding of the kinetics of biomass fast pyrolysis.