Radioactive molecular iodine (I2) and methyl iodide (CH3I) coexist in the off-gas stream of nuclear power plants at low concentrations and only few adsorbents can effectively adsorb low-concentration I2 and CH3I simultaneously. Here, the authors demonstrate simultaneous capture of I2 and CH3I at low concentrations by exploiting different adsorptive sites in a covalent organic framework.