The controlled incorporation of sodium into the absorber layer of CuInS2 solar cells improved cell performance remarkably. Without toxic KCN treatment, conversion efficiencies of over 6% were achieved by sulfurization of sodium-containing precursors, We also investigated the characteristics of the sodium-incorporated CuInS2 films by intentional addition and diffusion from a soda-lime glass. The ternary compound semiconductor of NaInS2 was found to form mainly on the surface of each of the CuInS2 films.