Pyrolysis of biomass produces both pyrolysis oil and solid char. In this study, poplar has been pyrolyzed in a stepwise fashion over a series of temperatures from 200 to 500 °C, and both the primary products contributing to pyrolysis oil and the changes in the pyrolyzing poplar surface leading toward char have been characterized at each step. The primary products were identified by direct analysis in real time (DART) mass spectrometry, and the changes in the poplar surface were monitored using Fourier transform infrared (FTIR) photoacoustic spectroscopy, with a sampling depth of a few micrometers. The primary products from pyrolyzing cellulose, xylan, and lignin under similar conditions were also characterized to identify the sources of the poplar products.