through crystallisation from the organic solution of form I in the presence of aspirin anhydride. A bulk solid mixture of 9 g aspirin and 1 g aspirin anhydride was prepared, and divided into 10 x 1 g portions in separate vials. The solid in each vial was dissolved in 3 ml tetrahydrofuran (THF) with shaking, then left to evaporate under ambient conditions. X-ray powder diffraction (XRPD) was used to confirm the crystalline structure of all samples prior to any subsequent terahertz spectroscopy experiments.