The disappearance of the two forms of glycolide suggests that the unknown product is related to this either as a new solid-state form of glycolide e.g. polymorph, hydrate etc. or a reaction product. Separate time-dependent stability studies of glycolide powder were performed in-house. Glycolide was stored as the pure phase and as the blended mixture under the same conditions as the tablets used in the study (5°C and 65% RH). The powder diffraction study (Figure ) shows that glycolide converts to the same unknown phase over the course of a few weeks. In both the pure and blended mixtures, the conversion to the unknown phase begins after two to three weeks but the extent of conversion appears to be accelerated in the blended product with complete conversion after four weeks where the conversion of the pure phase is complete after 10 months. No data were collected between 2 and 10 months due to assumed stability but 3.6% conversion had occurred at 2 months in the pure sample. The conversion of the pure phase enabled indexing of the sample with unit cell parameters of a = 5.28(5) Å, b = 5.35(5) Å, c = 19.92 (19) Å, β = 90.900(13)°, volume = 562(9) Å3 and space group P21/c. Unfortunately, the structure could not be solved from the powder diffraction data but the crystal quality was good enough to use the recent developments in electron diffraction to solve the crystal structure of the unknown phase. Full experimental details are included in section S1.6 of the supporting information. The unknown phase from our stability studies is 2-(2-hydroxyacetoxy)acetic acid (HAA), the hydrolytic ring opening product of glycolide (Figure ). The crystal structure from the ED data provides an excellent fit to the XRPD patterns collected as part of the stability study and as tablet, confirming phase purity (Figure ). HAA crystallizes in monoclinic space group P21/c with one molecule in the asymmetric unit. HAA molecules hydrogen-bond through O1-H…O2 H-bonded dimers with an R22(8) graph set notation and O5-H…O4/O5 bifurcated H-bond, forming a layered herringbone structure. To the best of our knowledge, this is the first time this structure has been reported.