The localization of Na impurities at the buried heterojunction of CdS/Cu(In,Ga)Se: thin film solar cells has been studied by photoelectron spectroscopy and X-ray emission spectroscopy. This combination of a surface- and a bulk-sensitive technique allows to identify the localization of impurities at a buried interface in a non-destructive. semi-quantitative. and element-specific way. We compare samples with increasing CdS-overlayer thickness on (a a CIGS film with nominal Na content and (b) a Na-lich CIGS film. The data clearly indicate a self-limitation of the Na content at this interface. The consequences are discussed in view of the possibility to tailor the electronic structure of the buried heterojunction by controlling the nominal Na content in the CIGS film.