X-ray powder diffraction measurements were carried out on a Panalytical XPert Pro diffractometer in Bragg-Brentano geometry, using non-monochromated CuK𝛼 radiation (𝜆ave = 1.5418 Å). Samples were prepared on glass flat-plate sample holders and data were measured over the range 2𝜃 = 3-40°. Due to the polytypic relationship between the crystal structures, the diffraction patterns of aspirin form I and II are closely comparable (50% of the reflections are identical). Form II domains are indicated by signature peaks at 2𝜃 ≈ 16.0, 19.2 and 26.0°. 5