In the present study, we investigated Cs-intercalated single wall carbon nanotubes (SWCNTs) using 133 Cs Nuclear Magnetic resonance. We show that there are two types of Cs cations depending on the insertion level. Indeed, at low concentrations, Static spectra analysis shows that the Cs (a)+ species are fully ionized, i.e. a equal ca.1, while at higher con-centrations a second paramagnetically shifted line appears, indicating the formation of Cs (b)+ ions with b < a $ +1. At low concentrations and low temperatures the Cs (a)+ ions exhi-bit a weak hyperfine coupling to the SWCNT conduction electrons, whereas, at higher tem-peratures, a thermally activated slow-motion diffusion process of the Cs (a)+ ions occurs along the interstitial channels present within the carbon nanotube bundles. At high con-centrations, the Cs (b)+ ions seem to occupy well defined positions relative to the carbon lat-tice. As a matter of fact, the Korringa relaxation behavior suggests a strong hyperfine coupling between Cs nuclei and conduction electrons in the carbon nanotubes and a partial charge transfer, which suggest a plausible Cs(6s)–C(2p) hybridization.