Abstract: Adsorption of Mo(VI) on 2−0.2-µm size fraction of sodium-saturated kaolinite at 25 ± 2°C and at a constant pH of 7.00 ± 0.05 was studied. The kaolinite sample was pretreated to remove any surface oxide and hydroxide coatings. The initial concentrations of Mo in solution ranged from 1 to 11 mg/liter in a NaClO4 background electrolyte at a constant ionic strength of 0.09 ± 0.01. Calculations of speciation using the GEOCHEM computer program indicated that under experimental conditions Mo(VI) was mainly in the MoO42− form. The experimental conditions were also shown to fulfill the requirements for applying the Langmuir equation in interpreting adsorption data. The Langmuir parameter for the adsorption maximum, n0 and the affinity parameter, KMoO42−-ClO4− were computed to be 3.33 × 10−4 mole/mole of adsorbent and 5.969 × 105, respectively. The large affinity parameter indicated that the Nasaturated kaolinite surface has a very high affinity for MoO42− ions relative to ClO4− ions.