This review critically summarizes currently known results concerning the thermal decomposition of the most frequently used ferrate(VI) salts (K 2FeO4, BaFeO4, Cs2FeO4). Parameters important in the thermal decomposition of solid ferrates(VI) include the initial purity of the sample, a presence of adsorbed and/or crystal water, reaction atmosphere and temperature, crystal linity, phase transitions, and secondary transformation of the decomposition products. The confirmation and identification of metastable phases formed during thermal treatment can be difficult using standard approach. The in-situ experimental approach is necessary in some cases to understand better the decomposition mechanism. Generally, solid ferrates(VI) were found to be unstable at temperatures above 200 °C as one-step reduction accompanied by oxygen evolution usually proceeds. The most known and used ferrate(VI) salt, potassium ferrate(VI) (K2FeO4), decomposes at high temperatures to potassium orthoferrate( III), (KFeO2), and potassium oxides. The resulting phase composition of the sample heated in air can be affected by accompanying secondary reactions with the participation of CO2 and H2O in air. However, the thermal decomposition of barium ferrate(VI) (BaFeO 4) is not sensitive to constituents of air and is mostly reduced to non-stoichiometric BaFeOx(2.5 < x < 3) perovskite-like phases stable under ordinary conditions. Such phases contain iron atoms with oxidation state +4; exhibiting the main difference in the decomposition mechanisms of K2Fe04 and BaFe04.