BACKGROUND Significant operating costs incurred in the liquid‐phase oxidation process using iron chelate catalytic solution (e.g. LO‐CAT) due to degradation and eventual loss of the iron chelate is a serious deficiency. Hydroxyl radicals formed in the Fenton reaction of ferrous chelate with H2O2, generated during the regeneration of the ferrous chelate with air, are responsible for iron chelate degradation. Although iron chelate degradation may be slowed down or inhibited by the catalytic decomposition of H2O2 into O2 and H2O, what is not yet clear is the impact of H2O2 decomposition by MnO2 on iron chelate degradation. In this work, the effects of H2O2 concentration, MnO2 concentration, oxygen as well as the initial pH value on iron chelate degradation in alkaline Fenton system of Fe2+‐EDTA/H2O2 (where EDTA is ethylenediaminetetraacetic acid) were studied. RESULTS Results showed that the percentage degradation of Fe‐EDTA increased with H2O2 concentration and decreased with H2O2 decomposition. In contrast to commercial MnO2, nanoscale MnO2 had much excellent catalytic activity on the decomposition of H2O2; thus 60 mmol L−1 nanoscale MnO2 inhibited Fe‐EDTA degradation by up to 82%. Furthermore, the inhibition in Fe‐EDTA degradation by H2O2 decomposition on nanoscale MnO2 was independent of pH from 6.0 to 8.5, and was related to oxygen concentration. CONCLUSION Nanoscale MnO2 can effectively inhibit the iron chelate degradation in LO‐CAT, and the significant operational costs associated with iron chelate degradation will be reduced by H2O2 decomposition on recyclable nanoscale MnO2 catalysts. © 2023 Society of Chemical Industry (SCI).