Zeolitic imidazolate framework-leaf (ZIF-L) is transformed to ZIF-8 through a topotactic phase transformation. This phase transformation is driven by the removal of the hydrogen-bonded linker molecules from 2D galleries of ZIF-L, leading to distortion in the hydrogen-bonded structure. Investigation of crystallization kinetics of ZIF-L shows that crystalline particles are formed within the first minute, and the growth of leaf-type structures is finished in nearly 10 minutes. During ZIF-L to ZIF-8 transformation, leaf-like ZIF-L particles are cleaved and transformed to 3D ZIF-8 nanocrystals. The chemical bonding structure investigated through Fourier transform infrared and Raman spectroscopy shows shifting and narrowing of characteristic peaks of stretching modes corresponding to linker molecules and Zn−N during the transformation. It confirms that distortions occur in the hydrogen-bonded structure of the ZIF-L phase. X-ray absorption spectroscopy shows modifications in the local structure around the Zn atom due to the reorientation of the fully bridged linker without affecting the Zn−N bond distances in the basic ZnN4 tetrahedral unit of ZIF-L or ZIF-8. Pore architecture evolution during the phase transformation is investigated by positron annihilation lifetime spectroscopy. During the transformation, the pores of ZIF-L are initially expanded with the decrease in their number density due to the breakdown of walls formed by intercalated linker molecules. With the increase in the ZIF-8 phase, the channel network of ZIF-8 sodalite topology along with inter-crystalline voids is gradually produced. A significant pore tuning of ZIF-L and ZIF-8 is achieved depending on the phase transformation time, which can be utilized for the enhancement in the gas separation efficiency of ZIF-8- or ZIF-L-based membranes.