Small angle x-ray scattering is utilized to directly examine the formation of clusters in supersaturated solutions of glycine, in an attempt to understand their role in the nucleation process. The results suggest that the majority of glycine molecules exist as dimers in aqueous solutions, and monomers in 13% ($v/v$) acetic acid-water mixtures. As the water and acetic acid–water solutions crystallize into $\alpha$ and $\gamma$ forms, respectively, the findings indicate a direct correlation between molecular self-association in solution and the polymorphic outcome.

• Received 18 October 2006

DOI:https://doi.org/10.1103/PhysRevLett.99.115702