The preparation and performance of mixed matrix membranes (MMMs) based on polybenzimidazole (PBI) and ZIF-8 nanoparticles of different average sizes (50, 70 and 150 nm) as filler are reported. MMMs containing 10 and 20 wt% of ZIF-8 were tested for H2/CO2 separation (pre-combustion CO2 capture) at 150 °C and feed pressures from 3 to 6 bar. The addition of ZIF-8 resulted in a clear improvement in membrane performance. Embedding 20 wt% of ZIF-8 resulted in a H2 permeability increase of six times and the H2/CO2 selectivity increased nearly by 55% compared to the bare PBI polymer membrane. Both permeability and selectivity improved as the filler size increased, due to the lower degree of agglomeration of the largest particles, that may be less active owing to their smaller external surface area. MMMs synthesized using dry 150 nm ZIF-8 filler showed a better performance than those containing wet filler. Apart from agglomeration concerns favoring wet filler handling as evidenced by infrared characterization, the MMM preparation with wet filler is simpler than with dry filler. Finally, the reproducibility of the membranes was confirmed by a European interlaboratory Round Robin test involving three different institutions.