A Cu₂ paddle wheel-based metal-organic framework, [Cu₂(trz-ia)₂] (trz-ia^2- = 5-(4H-1,2,4-triazol-4-yl) isophthalate), is investigated for hydrogen adsorption and hydrogen isotopologue separation. Its ultramicroporous structure with pore diameters ranging from 0.35 to 0.53 nm allows for strong interactions with dihydrogen molecules, resulting in steep H₂ uptake and heat of adsorption Q_ads = 9.7 kJ mol^-1. Notably, the hydrogen density inside the pores is 43.9 g L^-1 at 77 K and 100 kPa. Thermal desorption spectroscopy (TDS) after exposure to a H₂/D₂ mixture indicates dihydrogen isotopologue separation with a selectivity of S = 6 at 30 K and a high uptake of D₂. These findings are compared with numerous other metal-organic frameworks (MOFs) and related to their pore size.