Radioactive molecular iodine (I2) and organic iodides, mainly methyl iodide (CH3I), coexist in the off-gas stream of nuclear power plants at low concentrations, whereas few adsorbents can effectively adsorb low-concentration I2 and CH3I simultaneously. Here we demonstrate that the I2 adsorption can occur on various adsorptive sites and be promoted through intermolecular interactions. The CH3I adsorption capacity is positively correlated with the content of strong binding sites but is unrelated to the textural properties of the adsorbent. These insights allow us to design a covalent organic framework to simultaneously capture I2 and CH3I at low concentrations. The developed material, COF-TAPT, combines high crystallinity, a large surface area, and abundant nucleophilic groups and exhibits a record-high static CH3I adsorption capacity (1.53 g·g−1 at 25 °C). In the dynamic mixed-gas adsorption with 150 ppm of I2 and 50 ppm of CH3I, COF-TAPT presents an excellent total iodine capture capacity (1.51 g·g−1), surpassing various benchmark adsorbents. This work deepens the understanding of I2/CH3I adsorption mechanisms, providing guidance for the development of novel adsorbents for related applications.