that if Pu 3+ is to be described as 5f , then PuO 2 must be assigned as 5f . The spectroscopic evidence must be mainly probing the non-diffuse component of the 5f shell density. This is consistent with our calculations. The NPA occupations obtained with the PBEO functional (25% exact exchange, shown in Table ) reveals 5f occupations leading to n f = 5.26 (plus 0.16 from the diffuse 5f') for Pu 3+ (aq) vs. n f = 4.71 (plus 0.81 from diffuse 5f') for PuO 2 . See Table and Table in SI for additional details. The M 5 -edge HR-XANES spectrum of Pu 4+ (aq) is slightly shifted (0.2 ± 0.05 eV) to higher energy compared to the PuO 2 spectrum (cf. Figure ). This shift indicates that Pu in PuO 2 receives slightly more donation than Pu 4+ (aq) . This finding is also consistent with the theoretical results. The calculated data are n f = 4.71 for PuO 2 and 4.62 for Pu 4+ (aq) (without considering the diffuse regions of 5f). The O 2-ions in PuO 2 donate overall more strongly than the oxygen lone pairs in the aqua complexes. Also, the calculations show that a substantial part of the donation by O 2-in PuO 2 leads to delocalized 5f occupation, i.e., a 5f' occupation of 0.81, whereas the 5f' occupation is only 0.30 for Pu . See Table , 3 and and S5 for additional details. Note that the spectral features typical for Pu 5+ O 2 1+ (aq) and Pu 6+ O 2 2+ (aq) are not present in the spectra of PuO 2 and Pu 4+ (aq). This is clearly visible in Figure where the Pu M 5 -edge HR-XANES spectra of Pu 5+ O 2 1+ (aq) and Pu 6+ O 2 2+ (aq) are compared to the spectra of PuO 2 and Pu 4+ (aq).