The stability of a common interface used in organic photovoltaic cells, between the transparent electrode of Indium Tin Oxide (ITO) and a buffer layer of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is strongly influenced by the presence of humidity during processing, leading to significant migration of indium and tin species into the PEDOT:PSS layer. The interface was studied using neutral impact collision ion scattering spectroscopy (NICISS) and X-ray photoelectron spectroscopy (XPS), to determine migration of indium and tin into the polymer layer. It was found that the migration starts almost instantly after spin coating of the aqueous PEDOT:PSS solution and it reaches a saturation level within twenty four hours. The indium and tin were found always uniformly distributed over the sampling depth of almost one-third of the thickness of the PEDOT:PSS layer. Exposure to humidity following annealing resulted in the highest concentration (1.8 × 10-3 mol cm-3) of indium or tin species, corresponding to about one indium or tin moiety per 4.7 monomer units in the PEDOT:PSS. The maximum bulk concentration of indium is about two orders of magnitude higher after exposure to humid conditions compared to vacuum dried conditions. XPS measurements confirm the presence of both indium and tin in the PEDOT:PSS and the formation of salts with the metal ions as cations.