Abstract The sorption of U(VI) onto Opalinus Clay, a natural clay rock from Mont Terri, Switzerland, was investigated as a function of pH in the absence and presence of humic acid (HA) under aerobic conditions ([U(VI)]=1×10−6M, [HA]=50mg/L, I=0.1M NaClO4, S/L=4g/L, pH3–10). Due to leaching of Opalinus Clay, several competing ions are present in solution, where especially Ca2+ and CO32- ions affect the speciation of U(VI) and HA in solution and consequently, their sorption properties. From pH7.0 to 8.2, the Ca2UO2(CO3)3(aq) complex predominates U(VI) speciation and sorbs only weakly onto Opalinus Clay. In the case of HA, the CaHA(II) complex dominates HA speciation over a wide pH range. However, in the pH range 4.2–6.7, the presence of HA leads to the formation of the aquatic complex UO2(OH)HA(I), which decreases U(VI) sorption. In the pH range from pH7.5–10, HA seems to have no significant influence on the U(VI) sorption. The sorption characteristics of U(VI) on Opalinus Clay in the absence of HA were predicted using PHREEQC. Thereby, the investigation was focused on the influence of the different mineral binding sites on U(VI) sorption onto Opalinus Clay.