Molybdenum is an essential trace element for both plants and animals. Its environmental geochemistry is not well known, however. We have studied the pH-dependent binding of molybdenum(VI) to ferrihydrite and to acid organic soils using conventional batch experiments, chemical equilibrium modelling and EXAFS (Extended X-ray absorption fine structure) spectroscopy. The results show that both surface complexation to oxide surfaces and organic complexation need to be taken into account for a correct assessment of the mobility of molybdenum(VI) in acid soils. Adsorption to ferrihydrite is very strong at low pH; the binding patterns and the competition with phosphate can be simulated using the CD-MUSIC surface complexation model. However, the complexation to organic matter is substantial at low pH, causing strong sorption of molybdenum(VI) also in organic soils that do not contain a significant amount of Fe or Al oxide. Batch experiments with one Spodosol mor layer and one Histosol at pH 4 revealed that the binding of molybdenum(VI) was unaffected by competition from phosphate. Furthermore, the EXAFS studies on these organic soils showed that the added molybdenum(VI) formed a complex with the organic matrix with mean Mo-O/N distances of around 1.9 Å. The only known type of complexes with such Mo-O/N distances is hexavalent molybdenum with a coordination number of 6 or 7, normally with organic ligands binding through oxygen or peroxo ligands. At pH > 4.5 however, the adsorption of molybdenum(VI) decreased rapidly with increasing pH. In conclusion, the bioavailability and leaching of molybdenum(VI) in the surface horizons of soils is affected by the pH value, as well as by the presence of adsorbing components such as oxides and organic matter.