The interactions between adsorbed gas molecules within porous metal-organic frameworks are crucial to gas selectivity but remain poorly explored. Here, we report the modulation of packing geometries of CO2and C2H2clusters within the ultramicroporous CUK-1 material as a function of temperature. In-situ synchrotron X-ray diffraction reveals a unique temperature-dependent reversal of CO2and C2H2adsorption affinities on CUK-1, which is validated by gas sorption and dynamic breakthrough experiments, affording high-purity C2H2(99.95%) from the equimolar mixture of C2H2/CO2via a one-step purification process. At low temperatures (<253 K), CUK-1 preferentially adsorbs CO2with both high selectivity (>10) and capacity (170 cm3g−1) owing to the formation of CO2tetramers that simultaneously maximize the guest-guest and host-guest interactions. At room temperature, conventionally selective adsorption of C2H2is observed. The selectivity reversal, structural robustness, and facile regeneration of CUK-1 suggest its potential for producing high-purity C2H2by temperature-swing sorption.