In the 1950s and 1960s, Semenov’s theory of branched chain processes had become capable of quantitatively describing hydrogen combustion [1, 2], but the theory fit the experimental data only in the depth of the ignition peninsula. As reported in [3], Semenov and colleagues observed [1] a hydrogen combustion rate exceeding the theoretical expectations, and Azatyan and Aleksandrov detected [4] finite “anomalously deep burnups” near the ignition limit, which were tens of times those calculated. In his Nobel lecture in 1956, Semenov formulated the concept of heterogeneous chain propagation [5]. Therefore, Semenov and Azatyan proposed [3] to seek the cause of the discrepancy between the theory and the experimental data among possible heterogeneous reactions. A figure in Semenov and Azatyan’s report [3] shows that the additional contribution of the hypothetical heterogeneous chain propagation to the gas-phase component of the process in terms of finite burnups was only 10‐12%. To explain the discrepancy, a mechanism was put forward in which HO 2 radicals form on the surface and react with hydrogen atoms to produce hydroxyl radicals leaving the surface for the gas phase. However, the case in which the reaction H + HO 2 yields not hydroxyl radicals but a hydrogen peroxide molecule leaving the surface for the gas phase is apparently more advantageous from an energy standpoint; therefore, the assumed processes of heterogeneous branching or noticeable propagation of chains are unlikely. The purpose of this study was to reveal and refine the contributions of various heterogeneous chain propagation mechanisms to the gas-phase component of the process using analysis and targeted experiments. Heterogeneous chain propagation can be experimentally studied only by highly sensitive methods. We used resonance fluorescence spectroscopy of rarefied flames, whose sensitivity threshold ( 10 8 cm —3 ) was suf