The crystallization of two organic molecules, tetrahexil-sexithiophene (H4T6) and dibenzotetrathiafulvalene (DBTTF), into nano-sized cavities has been investigated by means of in situ grazing incidence X-ray diffraction measurements, during the solvent evaporation. Thanks to these real time experiments, the phase content has been determined from the onset of the first crystalline molecular assembly to the stable system. The correlation between the size of the nano-cavities and crystallization mechanism, in the complex systems proposed, has been established. The obtained results shows that the nano-confinement can induce organic molecules, for which polymorphs coexistence is otherwise difficult to avoid, to selectively crystallize in a single phase. The proposed system can be applied to a wide range of organic molecules and can be advantageously exploited in different application fields including organic …