The performance of mixed-matrix membranes (MMMs) based on Matrimid® and benzimidazole-linked polymers (BILPs) have been investigated for the separation CO2/N2 and the dependency on the filler porosity. BILPs with two different porosities (BILP-101 and RT-BILP-101) were synthesized through controlling the initial polymerization rate and further characterized by several techniques (DRIFTs, 13C CP/MAS NMR, SEM, TEM, N2 and CO2 adsorption). To investigate the influence of porosity, the two types of fillers were incorporated into Matrimid® to prepare MMMs at varied loadings (8, 16 and 24 wt.%). SEM confirmed the well dispersion of BILP-101 and RT-BILP-101 indicating their good compatibility with polymer matrix. The partial pore blockage in the membrane was verified by CO2 adsorption isotherms on the prepared membranes. In the separation of CO2 from a 15:85 CO2:N2 mixture at 308 K, the incorporation of both BILPs fillers resulted in an enhancement in gas permeability together with constant selectivity owing to the fast transport pathways introduced by the porous network. It was noteworthy that the initial porosity of the filler had a large impact in separation permeability. The best improvement was achieved by 24 wt.% RT-BILP-101 MMMs, for which the CO2 permeability increases up to 2.8-fold (from 9.6 to 27 Barrer) compared to the bare Matrimid®.