Adsorptive separation using narrow-micropore adsorbents has demonstrated the potential to separate hydrogen isotopes. In this work, we employed an isotope-responsive separation using cobalt formate. A D<sub>2</sub>-responsive third sorption step was revealed, and consequently, a noticeable difference was observed in the uptakes of D<sub>2</sub> and H<sub>2</sub>. This may have resulted from the additional space created for D<sub>2</sub> due to its dense packing, as DFT calculations revealed that cobalt formate possesses 2.26 kJ/mol higher binding strength for D<sub>2</sub> than for H<sub>2</sub>. The exploitation of this D<sub>2</sub>-responsive third sorption step renders a promising separation performance, with a D<sub>2</sub>/H<sub>2</sub> selectivity of up to 44 at 25 K/1 bar. Lastly, cobalt formate was synthesized on a gram scale here, which makes it a prospect for commercialization.