The Clay Mineral Society SWy-2 Na-montmorillonite clay was reacted for 90 days with 15 mM K⁺ at 200°C, the temperature/solution conditions of deep geologic repository near-field regions. X-ray diffraction of alteration products indicated that a minor amount of interstratification occurred in the clay. Hydrothermally altered samples sorbed stable Eu and ²³⁷Np almost as effectively as the unaltered clay. Average K_d values were 1.79 × 10² kg/L – 2.37 × 10⁵ kg/L (Euonto altered clay), 3.05 × 10² kg/L – 1.54 × 10⁶ kg/L (²³⁷Np onto altered clay), 3.55 × 10⁴ kg/L – 1.30 × 10⁶ kg/L (Euonto unaltered clay), and 1.04 × 10³ kg/L (²³⁷Np onto unaltered clay). Speciation models indicated that Eu(III) and Np(V) complexation was influenced by the presence of chloride and hydroxide, but the largest observed impact on Eu and Np(V) sorption was solution pH which altered the affinity of Eu and Np(V) for the mineral surface.