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We studied molecular sieving properties of zeolitic imidazolate framework-8 (ZIF-8) by estimating the thermodynamically corrected diffusivities of probe molecules at 35 °C. From helium (2.6 Å) to iso-C₄H₁₀ (5.0 Å), the corrected diffusivity drops 14 orders of magnitude. Our results further suggest that the effective aperture size of ZIF-8 for molecular sieving is in the range of 4.0 to 4.2 Å, which is significantly larger than the XRD-derived value (3.4 Å) and between the well-known aperture size of zeolite 4A (3.8 Å) and 5A (4.3 Å). Interestingly, because of aperture flexibility, the studied C₄ hydrocarbon molecules that are larger than this effective aperture size still adsorb in the micropores of ZIF-8 with kinetic selectivities for iso-C₄H₈/iso-C₄H₁₀ of 180 and n-C₄H₁₀/iso-C₄H₁₀ of 2.5 × 10⁶. These unexpected molecular sieving properties open up new opportunities for ZIF materials for separations that cannot be …