The heat-discharging kinetics of an iron-substituted Mnsub.2Osub.3/Mnsub.3Osub.4 redox pair subjected to long-term thermal cycling tests using a temperature swing process at high temperatures was investigated for next-generation concentrated solar power plants equipped with thermochemical energy storage. The heat-discharge mode kinetics for long-term thermal-cycled samples have never been reported. Additionally, comparisons of the heat-discharge mode kinetics for both long-term thermal-cycled and as-prepared samples have never been discussed. In terms of the reproducibility and sustainability of thermochemical energy storage, kinetic evaluations of samples with thermally stable morphologies subjected to long-term thermal cycling at high temperatures are important for next-generation solar thermal power plants. For the long-term thermal-cycled sample, the A2 model based on the Avrami-Erofeev reaction describes the discharging mode behavior in a fractional conversion range of 0-0.24, the contracting area (R2) model best fits in a fractional conversion range of 0.24-0.50, and the third-order (F3) model matches in a fractional conversion range of 0.50-0.70. For the as-prepared sample, the power-law (P2) model describes the behavior of the first part of the discharging mode, whereas the Avrami-Erofeev (A4) model best fits the last half of the discharging mode. The predicted theoretical models for both samples were compared with previous kinetic data.