The interaction between mass transfer and reaction in gas-liquid systems cannot be described by analytical formulae when second- or higher-order kinetics is considered. The purpose of this paper is to emphasize the usefulness of approximative perturbation solutions with an experimental case where fluxes are needed on both the ps and liquid sides of the interfacial film. For the familiar stagnant film model with second-order chemical reaction, we present approximate solutions in cases which are diffusion controlled for at least one of the reacting components. Some widely used approximations, like the pseudo-first-order assumption, are obtained as special cases. Analytical expressions are derived for both gas- and liquid-side fluxes, valid in the borderline reaction region. The approximate solutions are compared to numerical solutions of the original system.