Covalent organic frameworks (COFs) with atomically precise integration of scaffolds into 2D/3D topologies have shown great promise for separation applications. In this work, a porosity modulation approach was developed for the construction of 2D COFs for efficient volatile iodine adsorption. By rationally turning building blocks, significantly different iodine separation performance was obtained on COFs, where the best COF adsorbent exhibits a high-performance volatile iodine uptake of 4.0 g g(-1) at 75 degrees C and under ambient pressure and full pore access of iodine was achieved. Most importantly, the separation experiments reveal that iodine uptake capacities on COFs are not only determined by their pore volumes but also significantly affected by their intrinsic pore sizes. Both should be regulated simultaneously to work in concert when designing new COFs for the capture of volatile iodine.