In region 3 crystals of pure caffeine and the 1:1 cocrystal are in equilibrium with a solution of invariant composition E1, while region 4 describes the stability zone of the 1:1 cocrystal. Its skewed nature is indicative of the problems encountered in using solvent evaporation to crystallize the 1:1 cocrystal.19 The hatched area in Figure 5 shows the region where the 2:1 cocrystal appeared.