A pyrolysis reactor with optical access was developed to study the in-situ reactions taking place during the first stages of fast pyrolysis. All aspects of the reactor are documented, including sample insertion, temperature profiling, and species measurements. Analytical instruments including a gas chromatograph-flame ionization detector (GC/FID) and a Fourier transform infrared spectrometer (FTIR) were used in the experiments. In the case of the FTIR, a commercial instrument was modified to allow in-situ analysis and to enable sufficient sensitivity for time-resolved measurements during the reaction. Cellulose and Lignin were pyrolyzed, and qualitative identification of the main products was gained from the chromatogram with the FTIR as a function of residence time in the reactor. Also, hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) was applied to study phenol and 2,2'-biphenol demonstrating the capability of identifying biomass products using a non-invasive, spatio-temporally resolved in-situ technique. Finally, the optical access in the reaction chamber allowed direct time-resolved visualization of the pryrolysis process within the reaction zone. The data collected in this work illustrate that the implementation of in-situ diagnostic techniques can potentially provide new information on spatio-temporally dependent physicochemical processes that are relevant to biomass conversion.