<p>The present study systematically investigates the effect of reaction atmosphere on the non-isothermal thermal decomposition of iron(III) acetylacetonate [Fe(C₅H₇O₂)₃], a key organometallic precursor for iron oxide nanoparticle synthesis. Thermogravimetric analysis (TGA) was performed at three linear heating rates under nitrogen (N₂) and oxygen (O₂) atmospheres to elucidate the decomposition behavior and its kinetic–thermodynamic characteristics. Peak deconvolution of the differential thermogravimetric (DTG) profiles revealed a five-step process in N₂ and a six-step process in O₂. The activation energy (E_α), pre-exponential factor (A_α), and reaction mechanism functions were determined for each step using five integral iso-conversional methods—Flynn–Wall–Ozawa, Kissinger–Akahira–Sunose, Tang, Starink, and Vyazovkin—coupled with the master plot method. The estimated kinetic parameters exhibited strong dependence on the degree of conversion, confirming the multistep and complex nature of the decomposition. The activation energy was markedly higher for the later stages in N₂, whereas lower values in O₂ indicate facilitated decomposition under oxidative conditions. Thermodynamic parameters (ΔH, ΔS, and ΔG) derived from kinetic data suggest endothermic and non-spontaneous decomposition steps in both atmospheres, with larger positive entropy changes in O₂ implying increased molecular disorder during oxidation. X-ray diffraction (XRD) analysis of the final residues confirmed the formation of a mixed hematite–magnetite phase under N₂ and pure hematite under O₂. The comprehensive kinetic and thermodynamic evaluation demonstrates that the reaction atmosphere critically governs both the decomposition pathway and product phase evolution, providing mechanistic insight into the controlled formation of iron oxide nanoparticles from iron(III) acetylacetonate.</p>