Batch sorption experiments at fixed initial Np(V) concentration ( approximately 1X10 (super -6) M 237 Np), M/V ratio (4 g L (super -1) ), and ionic strength (0.1 molal NaNO 3 ) were conducted to determine the effects of varying pH and P CO2 on Np(V) sorption on SAz-1 montmorillonite. The results show that Np(V) sorption on montmorillonite is strongly influenced by pH and P CO2 . In the absence of CO 2 , Np(V) sorption increases over the entire pH range examined ( approximately 3 to approximately 10), with measured sorption coefficients (K D ) of about 10 mL g (super -1) at pH<6 to K D approximately 1000 mL g (super -1) at a pH of 10.5. However, for experiments open to atmospheric CO 2 (P CO2 = 10 (super -3.5) atm), Np(V) sorption peaks at KD approximately 100 mL g (super -1) at pH of 8 to 8.5 and decreases at higher or lower pH. A comparison of the pH-dependence of Np(V) sorption with that of Np(V) aqueous speciation indicates a close correlation between Np(V) sorption and the stability field of the Np(V)-hydroxy complex NpO 2 OH 0 (aq). In the presence of CO 2 and aqueous carbonate, sorption is inhibited at pH >8 due to formation of aqueous Np(V)-carbonate complexes. A relatively simple 2-site Diffuse Layer Model (DLM) with a single Np(V) surface complexation reaction per site effectively simulates the complex sorption behavior observed in the Np(V)-H 2 O-CO 2 -montmorillonite system. The good agreement between measured and DLM-predicted sorption values suggests that surface complexation models based on parameters derived from a limited set of data could be useful in extrapolating radionuclide sorption over a range of geochemical conditions. Such an approach could be used to support transport modeling and could provide a better alternative to the current use of constant K D values in performance assessment transport calculations.