Covalent organic frameworks (COFs) are crystalline polymers with covalent bonds in two or three dimensions, providing pores 1–5 nm in diameter. COFs are typically isolated as microcrystalline powders, which are unsuitable for many applications that would leverage their tunable structures, such as optoelectronic devices and nanofiltration membranes. Here, we report the interfacial polymerization of polyfunctional amine and aldehyde monomers with a Lewis acid catalyst, Sc(OTf)3. Immiscible solutions segregate the catalyst from the monomers, confining polymerization to the solution interface. This method provides large-area, continuous COF films (several cm2) with a thickness tuned from 100 μm to 2.5 nm. Relatively thick films were crystalline, whereas the films that are a few nanometers thick were presumably amorphous. The COF films were transferred onto polyethersulfone supports, and the resulting membranes showed enhanced rejection of Rhodamine WT, a model water contaminant. The large area, tunable pore size, and tailored molecular composition show promise for nanofiltration applications. Two-dimensional covalent organic frameworks (COFs) are crystalline polymers with grid-like structures. COFs show promise for applications such as energy storage devices and water-purification membranes. However, their typical microcrystalline, insoluble powder form complicates or precludes their use for these applications. Here, we have formed COFs at oil-water and air-water interfaces, which provide continuous films of these materials of arbitrary size and controlled thickness. These COF films can be transferred to both solid substrates and membrane supports, and preliminary composite membranes showed rejection of model organic pollutants. This approach indicates a way forward for accessing COF films on any substrate and will enable molecular design approaches to be rationally applied to nanofiltration membranes and other applications. Interfacial polymerization with COF monomers and Sc(OTf)3 afforded large-area (several cm2) free-standing films with tunable thickness (2.5 nm to 100 μm). When the films were thick (∼100 μm), they exhibited X-ray diffraction corresponding to the expected crystalline structure. The films were integrated into the thin-film composite membranes for water nanofiltration, where they showed enhanced rejection of model pollutant Rhodamine WT. © 2017