A gaseous mass transfer model has been proposed for quantitative evaluation of the chemical chain-back reaction system with volatile species in a boiling channel. Theoretical expressions for concentration transients in liquid and vapor phases were obtained. The model was applied to water radiolysis in a boiling water reactor core channel with Bankoff's two-phase flow treatment. Hydrogen injection tests in the Oskarshamn-2 and Dresden-2 units were simulated. The calculated results showed that gas release and absorption rates in the boiling channel were not consistent with Henry's law. By using optimized parameters related to the gaseous mass transfer, calculated results agreed within a factor of 2 for lower hydrogen injection rates at the two plants. It was determined that more exact treatments are needed to determine the radiation level in the downcomer and catalytic decomposition rate of hydrogen peroxide in order to provide better evaluations of water radiolysis phenomena.