Zeolitic imidazolate frameworks (ZIFs) have been foregrounded as structures with exceptional, intrinsic chemical and thermal stability. However, there has yet to be a systematic study of the isothermal stability of ZIFs, specifically the well-studied ZIF-8. In this work, ZIF-8 isothermal TGA decomposition kinetics were studied in air, argon, H2/CO2, and nitrogen environments by exposing ZIF-8 to each gas for 20 h at temperatures of 200, 250, and 300 °C, respectively. ZIF-8 crystallinity was preserved under the experimental isothermal conditions at 200 °C in each atmosphere, but crystallinity was increasingly eliminated at higher temperatures. Decomposition kinetics data show that the rate of ZIF-8 carbonization significantly increases at temperatures above 200 °C irrespective of environment. ZIF-8 decomposition in the H2/CO2 reducing mixture exhibits the slowest decomposition kinetics at all temperatures and the greatest morphological change. At 300 °C, oxidative effects enhance ZIF-8 decomposition in air. At lower temperatures the decomposition rate in air behaves more similarly to that of nitrogen and argon. Arrhenius activation energy parameters enable postulation that the temperature dependency of ZIF-8 thermal decomposition after carbonization at 300 °C is more similar upon decomposition in inert and reducing environments as compared to decomposition in oxidizing atmosphere. Four chemical equations inferring the residual carbonized ZIF structure after decomposition at 300 °C were developed based upon EDS quantification and FTIR/azirine formation models. The FTIR/azirine derived model postulates a heterogeneous carbonized ZIF-8 structure containing 2-methylimidazole and azirine rings coordinated to zinc and more precisely agreed with TGA weight decomposition data than the EDS derived model.