For the safe long-term storage of high-level radioactive waste (HLW), detailed information about geo-chemical behavior of radioactive and toxic metal ions under environmental conditions is important. Natural organic matter (NOM) can play a crucial role in the immobilization or mobilization of these metal ions due to its complexation and colloid formation tendency. In this study, the complexation of europium (as chemical homologue of trivalent actinides such as americium) and uranium (as main component of HLW) by ten humic acids (HA) from different sources and Suwannee NOM river extract has been analyzed. Capillary electrophoresis in combination with inductively coupled plasma mass spectrometry has been used for the evaluation of complex stability constants log beta. In order to determine the complex stability constants a conservative single site model was used in this study. In dependence of their source and thus of NOM structure the log beta values for the analyzed humic acids are in the range of 6.1-7.0 for Eu(III) and 5.2-6.4 for U(VI) (UO22+), respectively. In contrast to the results for HA the used Suwannee river NOM reveals log beta values in the range of nearly two orders of magnitude lower (4.6 for Eu3+ and 4.5 for UO22+) under the geochemical conditions applied in this study.