  Electron energy loss (EEL) spectra have been obtained on materials with the
nominal compositions of Na0.33CoO2_yH2O (y = 0, 0.6 and 1.3). Spectral analyses
revealed systematic changes in the low-loss energy region (< 10eV) and the core
losses(O-K, Co-L3,2) along with the water intercalation. The inter (intra)-band
transitions and the relevant plasmon resonance were found to be sensitive to
water content. Variations of the electronic structure and the Co valence states
during hydration have been discussed based on our experimental results.
