The purification that can be attained via crystallization is on the one hand governed by kinetic factors, such as the growth rate, and can on the other hand be limited by thermodynamic factors, such as the formation of a solid solution of the impurity with the product. For organic substances, these solid solubilities are difficult to establish, especially for low partial solubilities in the solid state. For the case of the crystallization of two epimers of a diastereoisomer, the formation of partial solid solutions with a respective solubility limited to approximately 5 % and 3 % has been shown using both DSC and XRPD measurements. The enthalpies of fusion of the solids are a strong function of the content of the opposite epimer. This can be interpreted with a disruption of bonds in the lattice, a disruption index of similar to 8 is found. First determinations of the crystal structure of one of the epimers suggest the formation of a solid solution, which occurs via the replacement of dimers of molecules that are bound by two hydrogen bonds involving the epimeric hydroxyl group and a vicinal phenoxy group. By replacing dimers, the lattice distortion might be limited.