Carboxylate (RCOO-) molecules with different terminal groups (R = H, CH3, (CH3)(3)C, HC=C, and CF3) adsorbed on an atomically flat TiO2(110) surface were observed by a scanning tunneling microscope (STM). The carboxylates were identified molecule by molecule as spots of different heights in microscope images obtained at positive sample bias voltage. The observed height of the carboxylates agreed with the spatial distribution of the unoccupied orbitals localized on each terminal group.