An analytical solution of the governing differential equation for gas absorption in laminar falling films with the first order homogeneous reaction and external gas phase mass transfer resistance is shown. The analysis is simplified by introducing a phenomenological mass transfer coefficient, Bi, that is a Biot number. An enhancement factor is defined as the ratio of the calculated absorption rates and the absorption rates found from a simple penetration model. It is shown also that external gas phase mass transfer resistances can significantly lower the absorption rates compared with those found when gas phase resistances are not present.