Abstract We investigated how biopulping modifies chemical and physical properties of wood and how these changes affect the properties of the resulting fiber. Mechanical and chemical testing revealed wood cell changes during 2 weeks of colonization by Ceriporiopsis subvermispora. Typical mechanical properties, such as modulus of elasticity and maximum load, tracked reductions in energy needed for mechanical refining to pulp. The data indicate the fiber saturation point of spruce increased from 29% to 42% during biopulping. At the same time, titratable acid groups increased up to 62%. Chemical analysis showed that oxalic acid esters were produced in the wood during biopulping in sufficient amounts to account for the increase in acid groups. The benefits of biopulping—energy savings and increased handsheet strength—as well as other physical property changes are consistent with the mechanism we propose: biopulping increases the acid group content of wood.