The sorption of (poly)molybdate ions into layered double hydroxides (LDHs), with Zn2+ and Al3+ cations, has been followed by in situ infrared spectroscopy using the attenuated total reflection technique. The exchange between solution molybdate species and interlayer anions has been followed in real time, illustrating the different behavior of molybdate ions and polymolybdate species. In a first part, the Mo(VI) speciation in solution was performed by comparison of thermodynamical calculations and infrared spectroscopy of solutions with different pH. Decomposition of bands between 800 to 1000 cm(-1), corresponding to the (Mo-O) stretching vibration, has permitted to identify major (poly)-molybdate species. In the presence of LDH, the measurements have shown a high affinity for heptamolybdate (Mo7O246-) species, and its preferential sorption in comparison with molybdate ions or other protonated polymolybdate species even if it represents very small fractions. From these measurements, the affinity series Mo7O246- > CO32- > MoO42- > SO42- have been directly obtained.