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In this chapter, a surface renewal model is developed for unsteady-state gas absorption in a stirred liquid contained in a batch cell, in which the dissolved gas undergoes a first-order chemical reaction with a liquid-phase reagent. Cases of zero and finite gas-phase mass-transfer resistance are considered in the model, which is applied to the absorption of oxygen in water containing a reagent like sodium sulfite. The model predicts that the rates of absorption and dissolved-gas transfer to the bulk liquid have an inverse behavior during the initial moments of absorption, the former declining rapidly from an extremely high value and the latter increasing smoothly from zero with the progress of time. As steady state is approached, these two rates level out with the former being higher than the latter due to the consumption of dissolved gas by reaction occurring in the surface elements. The predictions of the dissolved-oxygen concentration made with the rigorous model compare quite well with those of a much simpler pseudo-steady-state (PSS) model. Nevertheless, the rigorous surface renewal model provides a finer-grained picture of the phenomenon of absorption, that is, of the rates of absorption and dissolved-gas transfer during the initial period of absorption. A hybrid model that uses both the PSS and rigorous surface renewal models is proposed for performing unsteady-state gas absorption calculations.