Flexible metal-organic frameworks (MOFs) such as [Al(bdc)(OH)] (MIL-53) and Zn(C3H3N2) (ZIF-4) have attracted a great deal of attention recently due to their fascinating non-porous to porous phase transition upon the application of external stimuli such as temperature, hydrostatic pressure or gas pressure. Despite the large interest in these materials, lattice dynamic studies focusing on the origin of these phase transitions are rare. In a simple picture, there exists a balance in flexible MOFs between enthalpic effects (dispersion interactions) in the dense modification and entropic contributions (vibrational entropy) in the porous phase, which can be accessed by the external stimulus. To quantify the role of vibrational entropy and access the detailed origin of the phase transition, we here propose an inelastic neutron scattering study on a series of different flexible MOFs.