Polymers of intrinsic microporosity (PIMs) have gained significant research interest because of their successful applications in adsorption and separation. PIM-1 is the first and most studied member of this class because it shows specific interactions with some certain organic species. Chemical modification of PIM-1, which can be achieved by simply hydrolyzing the nitrile groups in the backbone, provides an advantage of tailoring its adsorption and separation performances. In this study, electrospinning of ultrafine fibers from hydrolyzed polymer of intrinsic microporosity (HPIM) and blends of hexamethylene diisocyanate (HMDI)/HPIM was achieved in several different ratios of HMDI/HPIM ranging from 1:9 to 1:1 (w/w). Bead-free and uniform fibers were obtained in the form of self-standing ultrafine fibrous membranes, which were then thermally treated at 150 °C to introduce chemical cross-linking between HMDI units and carbonyl groups of HPIM, resulting in HMDI-modified HPIM fibrous membranes (HMDI/HPIM-FMs). The solubility behavior has been altered by an introduced modification that makes membranes insoluble in all common organic solvents. Chemical cross-linking has been confirmed by using a Fourier transform infrared technique showing urethane linkage between HMDI and HPIM, and it was further supported by X-ray photoelectron microscopy and elemental analysis techniques that show a significant increase in the relative ratio of nitrogen in HMDI/HPIM-FMs compared to HPIM-FM. The average fiber diameters of fibrous membranes were found between 1.38 ± 0.29 and 0.96 ± 0.22 μm depending on the blend compositions and applied electrospinning parameters. Moreover, the water contact-angle value for HPIM-FM increased with the introduced HMDI modification from 140 ± 4° to 159 ± 7°, changing the nature of the membrane from hydrophobic to superhydrophobic. Consequently, HMDI/HPIM-FMs were successfully employed in oil/water separation due to the superhydrophobicity. In addition, the adsorption properties of HPIMFM and HMDI/HPIM-FMs were explored for common organic solvents. While both HPIM-FM and HMDI/HPIM-FMs show promising results, the structural stability of HMDI/HPIM-FMs in liquids was found to be more stable and reusable with respect to HPIM-FM. Hence, HMDI/HPIM-FMs are more favorable for organic adsorption and separation purposes from an aqueous system.