Covalent organic frameworks (COFs) are porous materials that offer new dimensions of design in both structure and chemistry. Engineering nanoscale COF materials into functional membranes with tunable properties is critical for adapting COFs to achieve challenging liquid separations that are important to the energy, environmental, and health fields. This presentation will describe the synthesis of ultrathin COF films and the fabrication of COF membranes. Large-area (64 cm2), ultrathin (24 nm), imine-linked COFs are synthesized using a facile interfacial polymerization technique. Angstrom-level control over single-digit nanopore size (1.4 nm – 2.0 nm) is achieved by direct integration of variable-length monomers. Tunable COF properties enable control over COF membrane mass transport, resulting in high solvent fluxes and sharp molecular weight cutoffs. The application of continuum models will be presented for quantifying the relative contributions of pore passage resistance to mass transport over pore entrance resistance. A strong linear correlation between single-digit nanopore tortuosity and COF thickness enables solvent fluxes to be predicted directly from the solvent viscosity and COF membrane properties. The contributions of solvent-nanopore interactions to transport behavior, as characterized by the membrane critical interfacial tension, will also be presented. The implications for rational design of COF membranes will be discussed.