In this paper we obtain an asymptotic development compatible with Freundlich’s isotherm, theta = KCm. That expression was derived underlying the theory of heterogeneous surface. We assume the distribution function of adsorption sites to be in the exponential form and the sites to follow the Langmuir energy homogeneous isotherm. The derived expression represents an analytical solution to the fundamental integral equation for the overall adsorption isotherm in its correct integration limit, From this expression we clearly identify the parameters K and m, the Freundlich parameters. We applied this expression to fit the experimental data for the adsorption of CO2 into BPL-activated carbon.