The bulk solids from the crystallisation trials contained differing degrees of form II domains, as indicated by the relative intensities of the signature form II peaks in the XRPD patterns (Figure ). Trace aspirin anhydride was also detected in most samples. The variability in the crystallization outcome is attributed principally to (deliberate) inhomogenous mixing of the initial aspirin/aspirin anhydride solid sample. Three samples were selected for further analysis, which contained minimum quantities of aspirin anhydride and a range of form II peak intensities.