We report on the electronic properties of Cs-intercalated singlewalled carbon nanotubes (SWNTs). A detailed analysis of the 13 C and 133 Cs nuclear magnetic resonance (NMR) spectra reveals an increased metallization of the pristine SWNTs under Cs intercalation. The 'metallization' of Cs x C materials where x = 0-0.144 is evidenced from the increased local electronic density of states (DOS) n(E F ) at the Fermi level of the SWNTs as determined from spin-lattice relaxation measurements. In

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... lar, there are two distinct electronic phases called α and β and the transition between these occurs around x = 0.05. The electronic DOS at the Fermi level increases monotonically at low intercalation levels x < 0.05 (α-phase), whereas it reaches a plateau in the range 0.05 x 0.143 at high intercalation levels (β-phase). The new β-phase is accompanied by a hybridization of Cs(6s) orbitals with C(sp 2 ) orbitals of the SWNTs. In both phases, two types of metallic nanotubes are found with a low and a high local n(E F ), corresponding to different local electronic band structures of the SWNTs.