Steam explosion pretreatment modifies both the chemical and physical structures of a biomass. Chemical modifications are generated during the treatment of biomass with steam at high temperature. Physical modifications are created during the explosion step, which produces disintegrated and defibrillated biomass. In this study, the contribution of each modification to an increase in enzymatic hydrolysis has been studied. It was found that both physical and chemical modifications contributed to an increase in enzymatic hydrolysability. Additionally, high resolution X-ray tomography was performed to identify the structural modification created during the steam explosion process. Comparison of the 3-D micro-structure of a steam-exploded wood sample with an untreated wood sample revealed that several kinds of cracks were created during the explosion step, and the micro-structure of the wood sample was vigorously destroyed.