The relevant length-scale and rate of intra-particle diffusion limits the minimum time required for complete acid impregnation, which in turn affects pretreatment efficiency.  Biomass theoretically modeled as spheres is ubiquitous in the literature and a natural first-order approximation.  However, plant structure tends to be planar in nature.  Here, we used turned wooden spheres to validate the assumption of uniform radial distribution in spherical particles so often used when modeling biomass.  Images of pretreated sphere sections are compared to predicted diffusion profiles. Energy-dispersive X-ray Spectroscopy (EDS) coupled with Scanning Electron Microscopy (SEM) reveal details of the sulfuric acid penetration into the wood structure.  We explore the transport profiles for saturated spheres and those with incomplete acid impregnation for a range of particle sizes and compare them to pretreatment effectiveness.  Sphere fragmentation occurs only during high-severity pretreatments, thus it is not probed here.