We compared four types of ZIF-8 with varying sizes and shapes to determine their thermal-structural stability and derive appropriate thermal activation conditions and correlation between structural characteristics and adsorption properties. Under air, the ZIF-8 phase for all the samples was converted completely into the zinc oxide phase above ∼300°C, though thermalgravimetric analysis (TGA) indicated that the original structure was stable to ∼300-350°C. Longer exposures (∼30 d) suggested that thermal activation at ∼200°C was appropriate for the removal of guest and/or solvent molecules under air without structural damage. Despite no noticeable change in X-ray diffraction (XRD) patterns after activation at 250°C under air, the resulting BET surface areas and CO2 adsorption amounts (at 1 bar and 30°C) of ZIF-8s were reduced to ∼44-54 and ∼72-87%, respectively, as compared to those of appropriately activated ZIF-8s. It appears that after the activation at 250°C under air, some Zn and N atoms were dissociated and converted to ZnOH and NOH, respectively, causing the partial structural damage of ZIF-8s.