Gas absorption in a laminar falling film with instantaneous chemical reaction with a reactant in solution has been theoretically analysed using a boundary immobilization technique. The profile of the reaction front, concentration distributions, the dimensionless quantity of gas absorbed and the enhancement factor have been computed for different values of the diffusivity ratio D and the concentration ratio β. Although the concentration distribution of the dissolved reactant is dependent on D and β, that for the dissolved gas is nearly independent of these parameters for small depth of penetration. The enhancement factor shows a convex profile for small D, an explanation for which has been provided. Copyright © 1986 Canadian Society for Chemical Engineering