  We report a complete analysis of the formal oxidation state of Co in NaxCoO2,
in the interval 0.31<x<0.67. Iodometric titration and thermoelectric power
confirm that a direct relationship between the Na content and the amount of Co
3 + cannot be established in this system. Creation of a significant amount of
oxygen vacancies accompanies Na-ion deintercalation, keeping the formal Co
valence at 3.45 + for x<0.45. To the light of new thermoelectric power data
which reveals important differences between the hydrated (superconducting) and
non-hydrated (non-superconducting) samples, we propose here that water plays an
important chemical role beyond that of a spacer between the CoO2 layers.
