The structural complexity of unprocessed plant tissues used for thermochemical conversion of biomass to fuels and energy impedes heat and mass transfer and may increase the occurrence of tar-forming secondary chemical reactions. At industrial scales, gas and liquid products trapped within large biomass particles may reduce net fuel yields and increase tars, impacting industrial operations and increasing overall costs. Real-time microscopic analysis of poplar (Populus sp.) wood samples undergoing anoxic, pyrolytic heat treatment has revealed a pattern of tissue and macropore expansion and collapse. Post-reaction Structural analyses of biomass char (biochar) by light and transmission electron microscopy have provided direct structural evidence of pyrolysis product mass-transfer issues, including trapped pyrolysis products and cell wall compression, and have demonstrated the impact of heat-transfer problems on biomass particles. Finally, microscopic imaging has revealed that pyrolyzed/gasified biochars recovered from a fluidized bed reactor retain a similar pre-reaction basic plant tissue structure as the samples used in this study, suggesting that the phenomena observed here are representative of those that occur in larger scale reactors.