The mechanism of monotonous decomposition of hydrogen peroxide by iodide in acidic medium is investigated at room temperature. For this purpose, O-2 pressure, I-, I-2, and I-3(-) concentrations are simultaneously monitored, establishing a useful framework for better understanding of the reaction mechanism and testing various models. The possibility of nonradical and radical approaches to describe experimental data is examined. Results suggest that the best description of experimentally recorded components is obtained by introducing free radicals into the model of the reaction.