Abstract: This work shows the possibility of formation of membranes with an asymmetric porous structure based on copolyimide/nanodiamonds (coPI/ND) composites. The influence of nanodiamonds as well as variation of the nature of the solvent by introducing an ionic liquid on the structure and thermal properties of the membranes is investigated by X-ray diffraction analysis, thermogravimetric analysis, scanning electron microscopy, and IR spectroscopy. The transport properties of the membranes are investigated in the process of ultrafiltration of aqueous solutions of proteins. The effect of the additions of an ionic liquid on the optimization of transport characteristics is carried out. It is shown that membranes based on coPI/ND composites possess increased hydraulic performance: the introduction of up to 3 wt % ND and use of an ionic liquid as a cosolvent lead to increase in the flux up to 600 L m ⁻² h ⁻¹ bar ⁻¹ compared to a membrane based on PA.