[en] Under oxidising conditions, Technetium-99 will exist as pertechnetate (TcO₄^-), a highly soluble species, which does not interact with minerals or sediments. However under reducing conditions and when suitable electron donors (eg. Fe(II)-containing minerals) are present, Tc(VII) will be reduced to Tc(IV). Depending on the total concentration, the reduced Tc(IV) will precipitate to TcO₂.nH₂O. When complexing agents are present, such as humic substances, the soluble Tc(IV) species can be associated with these humic substances and this process will result in an enhanced Tc solubility. The reduction of Tc(VII) to Tc(IV) was previously demonstrated to be catalysed by Fe²+ present in reducing surfaces. The association between reduced Tc and humic substances was investigated in presence of pyrite as the reducing surface, and was explained by a complexation reaction mechanism given by the following equation: Tc_(IV)O²⁺ + HS <=> Tc_(IV)O²⁺-HS. K^HS-values of the order of 10¹⁸-10²⁰ were obtained. In this contribution we further elaborate on the influence of the nature of the reducing surface, the nature of organic matter and the distribution of Tc between humic and fulvic acids, and on the correctness of the aforementioned complexation reaction mechanism, by changing parameters such as pH, amount of humic substances and initial Tc concentration. For this purpose mainly Fe₃O₄ was used to exclude eventual interferences from S originating from surfaces such as FeS₂ and FeS. (orig.)