We report on measurements of the magnetic susceptibility χ(T) and 23Na nuclear magnetic resonance (NMR) on polycrystalline samples of Na 0.7 CoO 2 . The χ(T) data suggest that for T>75K, the Co ions adopt an effective configuration of Co3.4+. The temperature evolution of 2D exchange 23Na-NMR spectra and of the 23Na spin-lattice relaxation time T1 reveal significant anomalies between 230 and 300K. These features provide evidence for a crossover phenomenon or a phase transition and suggest that, upon decreasing temperature, a dramatic change occurs in the Co 3d–electron spin dynamics, starting just below 300K and being completed at 230K. This phenomenom may involve a partial charge ordering within the Co subsystem.