The hydrogenation of benzene to cyclohexadiene on a Ni(111) surface has been studied using ab-initio local-density-functional calculations. The calculations were performed using the Vienna ab-initio simulation package (VASP), which is based on a plane wave basis set and PAW potentials. Starting from benzene preadsorbed on the nickel surface reaction pathways for the hydrogenation of the benzene molecule to cyclohexadiene by a Langmuir-Hinshelwood mechanism have been investigated. In the optimized reaction path, the initial step is a dissociation of the hydrogen molecule over a nickel surface atom. The rate-determining step has been identified as the first hydrogenation step, i.e., the formation of C6H7, with an energetic barrier of 0.7 eV. Parallel to that, the direct interaction of the hydrogen molecule with the adsorbed molecule in an Eley-Rideal reaction has been studied. The Eley-Rideal reaction leads to a much higher barrier. A detailed analysis of the structural and electronic properties of the molecule at the corresponding transition states is presented.