Metal-organic frameworks (MOFs) were supported on polymer membrane substrates for the fabrication of composite polymer membranes based on unmodified and modified polymer of intrinsic microporosity PIM-1. Layers of two different MOFs, ZIF-8 and HKUST-1, were grown onto neat PIM-1, amide surface-modified PIM-1 and HMDA surface-modified PIM-1. The surface-grown crystalline MOFs were characterized by a combination of several techniques, including powder X-ray diffraction, infrared spectroscopy and scanning electron microscopy to investigate the film morphology on the neat and modified PIM-1 membranes. The pure gas permeabilities of He, H2, O2, N2, CH4, CO2 were studied to understand the effect of the surface modification on the basic transport properties and evaluate the potential use of these membranes for industrially relevant gas separations. The pure gas transport was discussed in terms of permeability and selectivity, highlighting the effect of the MOF growth on the diffusion coefficients of the gas in the new composite polymer membranes. The results confirm that the growth of MOFs on polymer membranes can enhance the selectivity of the appropriately functionalized PIM-1, without a dramatic decrease of the permeability.