This work presents a detailed comparison of the electronic properties of two phases (alpha and gamma) of the prototypical Si(111)- root 3 x root 3-Sn system, focussing on the empty surface states. The latter reconstruction has been studied at room temperature with inverse photoemission (KRIPES). For the gamma-phase characterised by an equal amount of Sn and Si adatoms, a charge transfer towards the Sn adatom leaves an empty surface state localised on Si adatoms, and observed in KRIPES 0.4 eV above the Fermi level (E-F) at the surface Brillouin zone (SBZ) center. The band gap of this semiconducting gamma-phase is estimated around 0.5 eV. In contrast, the alpha phase with only Sn adatoms shows a metallic behaviour and two distinct adatom-derived empty surface states.