The hydrogenation and dehydrogenation of benzene on Pt(1 1 1) is examined from first principles using DFT-GGA cluster calculations. The reactive benzene species is adsorbed at the hollow site. The addition of the first H-atom has a barrier of 74 kJ/mol and is +11 kJ/mol endothermic. There are five different pathways available for the addition of the second hydrogen atom. The dominant path is the one that forms the 1,3-dihydrobenzene intermediate. This reaction has a barrier of 72 kJ/mol and is +34 kJ/mol endothermic. The hydrogenation of the C6H7* intermediate can also form 1,3-cyclohexadiene, which has a barrier of 91 kJ/mol and is +38 kJ/mol endothermic, or 1,4-cyclohexadiene, which has a barrier of 115 kJ/mol and is +36 kJ/mol endothermic. Two types of hydrogenation mechanisms were distinguished. The "three-centered" mechanism was found to be more favorable than the "slip" mechanism. The dehydrogenation of benzene to phenyl is +76 kJ/mol endothermic. Therefore benzene dehydrogenation is neither thermodynamically nor kinetically a favorable reaction path. Dehydrogenation to o-benzyne is +14 kJ/mol endothermic relative to benzene. The calculated barriers are in qualitative and quantitative agreement with experimental data.