It is known that the ion-exchange process in glass is described by a non linear diffusion equation whose solutions represent the index profile of the waveguide formed. The diffusion equation depends on the relative mismatch of the incoming and original ion mobilities, characterized by the parameter (alpha) . If they are equal, the diffusion equation becomes linear, and its solutions are normally used as an approximation for non-linear processes. Nevertheless, empirical solutions often provide a better modelization, but they must be found in each particular case of diffusion. In this work, we develop a perturbative method for solving the non linear equation, thus approximated analytical solutions can be obtained up to intermediate (alpha) -values ((alpha) equals 0.5). We have applied it to a simple surface thermal waveguide. By another hand, a proper dependent variable change is performed in the diffusion equation, and then the perturbative method is applied. It provides a solution much more exact that can be used with great accuracy up to (alpha) equals 0.8, which is a situation where Gaussian functions are normally used as empirical solutions. The method can be applied to more complex situations as buried waveguides, field-assisted processes and so on.