*3.1. Preparation of Venetoclax Hydrate*

In our previous studies on the stability and liquid chromatography analytical method development for venetoclax, we observed that venetoclax formed crystals after a few days from some of the solvents used for the dissolution of venetoclax [59,60]. Therefore, we performed a targeted crystallization experiment from the most promising solvent system identified in our previous study [60]. For this purpose, venetoclax was dissolved in an ACN–NH4HCO<sup>3</sup> (10 mM solution) = 8:2 solvent mixture, and the solution was left to stand for 60 days at ambient temperature in a laboratory glass bottle. The yellow needle-shaped crystals that were formed on the walls of the glass bottle after evaporation of the solvent were collected and used for further analysis. This proved that the obtained crystalline venetoclax was suitable for single crystal X-ray diffraction. Thermal analysis and X-ray data indicated that the obtained crystals represented a venetoclax hydrate form that was previously reported in the patent literature, although it was obtained by desolvation of the venetoclax ethyl acetate solvate at ambient conditions and characterized only with a *p*-XRD [42].
