Solid orthorhombic crystals of potassium ferrate(VI) (K2FeO4) of a high-chemical purity (> 99.0%) were characterized by low-temperature (1.5-5 K), high-temperature (463-863 K), and in-field (1.5 K/3 T) Mossbauer spectroscopy. Potassium ferrate(VI) reveals a Neel magnetic transition temperature (T-N) of similar to 3.8 K and a saturation hyperfine magnetic field of 13.8 T at 1.5 K. Spectral line intensities recorded below T-N in an external magnetic field of 3 T manifest a perfect antiferromagnetic ordering. For the in situ monitoring of the thermal behavior of K2FeO4, high-temperature Mossbauer data were combined with those obtained from thermogravimetry, differential scanning calorimetry, and variable-temperature X-ray diffraction measurements. Such in situ approach allowed the identification of the reaction products and intermediates and yielded the first experimental evidence for the participation of CO2 in the decomposition process. As the primary conversion products, KFeO2 and two potassium oxides in equivalent molar ratio, KO2 and K2O, were suggested. However, the KO2 phase is detectable with difficulty as it reacts very quickly with CO2 from air resulting in the formation of K2CO3. The presented decomposition model is consistent with thermogravimetric data giving the mass loss of 8.0%, which corresponds to the participation of (1)/(6) mol of CO2 and liberation of (3)/(4) mol of O-2 per 1 mol of K2FeO4 (K2FeO4 + (1)/6CO2 -> KFeO2 + (1)/3K2O + (1)/6K2CO3 + (3)/O-4(2)). An explanation of the multistage reaction mechanism has an important practical impact for the optimization of the solid-state synthesis of potassium ferrate(VI).