In recent years, the peroxidase enzymes have generated wide interest in several industrial processes, such as wastewater treatments, food processing, pharmaceuticals, and the production of fine chemicals. However, the low stability of the peroxidases in the presence of hydrogen peroxide (H₂O₂) has limited its commercial use. In the present work, the effect of H₂O₂ on the inactivation of horseradish peroxidase (HRP) was evaluated. Three states of HRP (E₀, E₂, and E₃) were identified. While in the absence of H₂O₂, the resting state E₀ was observed, in the presence of low and high concentrations of H₂O₂, E_2, and E₃ were found, respectively. The results showed that HRP catalyzed the H₂O₂ decomposition, forming the species E_x, which was catalytically inactive. Results suggest that this loss of enzymatic activity is an intrinsic characteristic of the studied HRP. A model from a modified version of the Dunford mechanism of peroxidases was developed, which was validated against experimental data and findings reported by the literature.