X-ray scattering techniques [in situ resonant anomalous X-ray reflectivity (RAXR) and specular crystal truncation rods (CTR)] were used to compare muscovite (001) surfaces in contact with solutions containing either 0.1 mM plutonyl(VI) or 1 mM uranyl(VI) at pH = 3.2 +/- 0.2, I(NaCl) = 0.1 M, as well as in situ grazing-incidence X-ray absorption near-edge structure (GI XANES) spectroscopy and ex situ alpha spectrometry. Details of the surface coverage are found to be very different. In the case of Pu, alpha spectrometry finds a surface coverage of 8.3 Pu/A(UC) (A(UC) = 46.72 angstrom(2), the unit cell area), far in excess of the 0.5 Pu/A(UC) expected for ionic adsorption of PuO22+. GI XANES results show that Pu is predominantly tetravalent on the surface, and the CTR/RAXR results show that the adsorbed Pu is broadly distributed. Taken together with previous findings, the results are consistent with adsorption of Pu in the form of Pu(IV)-oxo-nanoparticles. In contrast, uranyl shows only negligible, if any, adsorption according to all methods applied. These results are discussed and compared within the context of known Pu and U redox chemistry.