Zirconium and hafnium behave nearly identically in most geological processes due to their identical nominal ionic charge and similar radius. Some of the most pronounced exceptions from this rule are observed in fluoride-rich aqueous systems, suggesting that aqueous fluoride complexation may be involved in Zr/Hf fractionation. To understand the mechanisms causing this phenomenon, we investigated complexation of Zr4+ and Hf4+ in fluoride-rich (1.0 mol/kg HF) aqueous solutions at 40 MPa and 100 - 400 °C, using synchrotron X-ray absorption spectroscopy (X-ray absorption near edge structure and extended X-ray absorption fine structure) combined with classical and ab initio molecular dynamics simulations. Below 200 °C, the observed complexes are [ZrF2(OH)2·2H2O]0 and [HfF2(OH)2·2H2O]0, respectively. The first coordination shell comprises a distorted octahedron, with fluoride and hydroxide ligands at a simila mean radial distance (ca. 1.85-1.95 Å) from the central cation, and H2O ligands at a slightly greater distance (ca. 2.1-2.2 Å). With increasing temperature, the H2O ligands move further out, causing first an increasing distortion of the octahedron and subsequently a partial transition to tetrahedral [ZrF2(OH)2]0 and [HfF2(OH)2]0 complexes as a certain fraction of the H2O molecules move to the second shell at > 3 Å. As a consequence, the radial distance of the F- and OH- anions from the central cation, as well as the overall average radial distance of the first shell decreases due to decreased steric repulsion from the H2O ligands. The fraction of tetrahedral versus octahedral complexes is higher for Hf than Zr between 200 and 300 °C. The results allow to postulate two possible mechanisms for Zr/Hf fractionation during precipitation of minerals from fluoride-rich hydrothermal solutions: 1) the portion of Zr and Hf residing in tetrahedral complexes experience mass-dependent fractionation during incorporation into higher coordinated sites in the solids, similar to mechanisms known from isotope fractionation; or 2) the portion of Zr and Hf that resides in octahedral complexes preferentially fractionates into the solids compared to the tetrahedral portion because it requires less bond reorganization, and because the octahedral portion is larger for Zr than for Hf, this leads to fractionation. In the former mechanism Zr/Hf fractionation would be fundamentally caused by the mass difference between the two elements, whereas in the latter coordination differences due to different cation-ligand bond-strengths for Zr and Hf would be the underlying cause.