Two-photon photoemission is used to investigate the interfacial band structure of the methanethiolate/Ag(111) interface. Two adsorbate-induced electronic states are observed: one occupied, the other an unoccupied excited state. The initially occupied and unoccupied electronic states have effective masses of -2m(e) and 0.5m(e), respectively. At a low coverage of thiolate molecules, the unoccupied state is measured to be nondispersive. At intermediate coverages both nondispersive and dispersive unoccupied electronic states are observed. This coverage dependent evolution can be interpreted as a phase transition from a less dense to a more dense layer configuration.