The adsorption of benzene on Pt(111) was analyzed using first-principles density functional theoretical cluster and periodic slab calculations. The preferred adsorption site at low coverage is the bridge(30) site with an adsorption energy of 117 kJ/mol. At the bridge(30) site, two of the C Pz orbitals are well aligned for overlap with the metal dz2 and dyz orbitals, leading to a strong C-Pt bond and a strong adsorption energy. The molecule'S second important site is the hollow(0) site with an adsorption energy of 75 kJ/mol. Comparing calculated and experimental vibrational frequencies confirms the preference for the bridge site at low coverage and also indicates that adsorption at the hollow(0) site becomes preferred at higher coverage. Adsorption at the hollow(30), the bridge(0) and at the atop sites was found to be unfavorable.