Abstract Two samples of oxidized sodium cobalt oxide, Na x CoO 2, where x = 0.3 and 0.7, have been studied by variable-temperature wide-line 23Na-NMR. All spectra exhibit a substantially paramagnetic-shifted response characterized by 2nd-order quadrupole broadening and a diamagnetic-shifted weaker response. The paramagnetic shifts are attributed to spins localized on cobalt atoms. The 2nd-order quadrupolar splitting is resolved for x = 0.7; whereas the lineshapes obtained for x = 0.3 do not show the resolution and indicate a more symmetric sodium environment and/or a greater degree of ion mobility. Lineshapes for the two compositions behave somewhat differently as the temperature is changed, although both samples exhibit gradual line-narrowing over the entire range (−150 to 100 °C). Spin-lattice relaxation time measurements reflect the presence of inequivalent sodium environments, with one distribution of sodium environments characterized by long relaxation times (10 ms–200 ms) and another set of environments characterized by much shorter times (< 500 μs).