Radioactive technetium-99 (⁹⁹Tc) is present in nuclear and medical waste. Its immobilization by magnetite (Fe(II)Fe(III)₂O₄) has been studied in the last decades, showing that magnetite reduces pertechnetate (Tc(VII)O₄⁻) to Tc(IV), which is either incorporated into the magnetite structure or forms Tc(IV)-Tc(IV)-dimers. The distribution between both phases as well as the incorporation mechanism remain, unclear. This work investigates the molecular environment of Tc after putting it in contact with presynthesized nanoparticulate magnetite as a function of pH (2 - 13) and time (up to 7 weeks). X-ray absorption spectroscopy (XAS) was combined with density functional theory methods (DFT) to decipher the mechanism of Tc(IV) incorporation. We observed that sorption of Tc(IV)-Tc(IV)-dimers initially occurs at pH 5 and pH 7, while Tc(IV) incorporation in magnetite prevails at longer times and at pH 10. We suggest that Tc(IV)-Tc(IV)-dimer sorption on magnetite is due to maghemitization, whereas Tc(IV) incorporation is due to the electron transfer from sorbed Fe2+ through magnetite and subsequent release of Fe(II) in solution (redox conveyor belt model), “burying” Tc(IV) into the magnetite structure. DFT calculations indicate that Tc(IV) incorporates in magnetite by an exchange of two Fe(II) atoms for one Tc(IV), keeping the charge balanced by creating a vacancy.