Small-angle neutron scattering (SANS) was used to study the porosity of pine samples before and after ionic liquid (IL) pretreatment. Pine samples were pretreated using 1-ethyl-3-methylimidazolium acetate ([C2C1Im][OAc]) at 110 degrees C for 3 h at biomass concentrations of 5, 10, 15, 20, and 25 wt % and at 130 degrees C for 3 h at biomass concentrations of 5 and 25 wt %. For the first time, relative changes in porosity of pretreated pine samples derived from SANS were compared with those obtained from the nitrogen adsorption analysis. Biomass crystalline structures were measured by X-ray diffraction (XRD). Both porosity and XRD data suggest that [C2C1Im] [OAc] interacted with pine samples more efficiently at higher biomass concentrations during pretreatment. This was attributed to the presence of resin acid in the pine samples.