The moving-boundary problem of gas absorption accompanied by an instantaneous irreversible chemical reaction in a limited gas-liquid system has been analyzed. Analytical solutions have been derived, under the restrictions of equal diffusivities of the two reacting species and stoichiometric factor of unity, for the cases of significant or negligible gas-film and surfactant-film resistances. A numerical scheme has been developed for the general case of unequal diffusivities and/or stoichiometric factor differing from unity. The effects of various parameters like film thickness, gas-bubble volume, stoichiometric factor, initial concentration in the liquid phase, etc., have been illustrated. Furthermore, the importance of the surfactant-film and gas-film resistances has also been demonstrated. The analytical solution has been found to be in agreement with the numerical solution, thus demonstrating the efficacy of the concept of negative concentration in solving such problems. (C) 2000 Elsevier Science S.A. All rights reserved.