Fine tuning of the pore architecture and flexibility of zeolitic imidazolate frameworks (ZIFs) is highly crucial for realizing their applications in molecular gas separation. Mixed ligand frameworks (ZIF-7-8) synthesized by mixing 2-methylimidazole (2meIm) and benzimidazole (bIm) ligands show enhanced gas separation performance, attributable to pore and flexibility tuning. In the present study, positron annihilation lifetime spectroscopy (PALS) measurements under CO2 pressure have been used to experimentally investigate the tuning of the pore architecture and flexibility of mixed ligand frameworks ZIF-7-8 having a ZIF-8 structure and similar morphology with varying bIm content up to 18.2%. The aperture and cavity of frameworks begin to open up with an increasing bIm ligand content followed by a decrease at a higher content. On the contrary, flexibility of the frameworks indexed from PALS measurements carried out under CO2 pressure shows a decreasing trend followed by an increase. The present study shows that mixed ligand frameworks having a larger aperture size are less flexible as a result of inherent open configurations of ligands in the framework lattice. On the other hand, frameworks having a comparatively smaller aperture size show higher flexibility as a result of a possibility of twisting of the ligands under CO2 pressure, resulting in aperture opening. The pore-opening phenomenon as a result of lattice flexibility under CO2 pressure is observed to be fully reversible for ZIF-7-8.