The effect of varying the hydrogen partial pressure (from 0.19 to 0.96 atm) on turnover frequencies (TOFs) in the gas-phase hydrogenation of benzene, toluene, and o-, m-, and p-xylene over a Ni/SiO 2 catalyst has been studied. Each system is characterized by well-defined reversible temperature related activity maxima (T max ) where T max was shifted in the conversion of benzene and toluene to lower values as the hydrogen partial pressure was reduced but remained unaffected in reactions involving the isomers of xylene. The range of TOFs reported for each aromatic system and the occurrence of T max are explained on the basis of interplay between the supply and reactivity of active surface species. The observed relationships between reaction temperature and TOF are represented by means of a common extended power rate model. The nature of true and apparent reaction kinetics is discussed, and a compensation effect is established wherein the only reaction variable is the hydrogen partial pressure. The experimentally determined activation energies do not represent the true catalytically relevant energetics because of the existence of preequilibria to the rate-determining step. The measured Arrhenius parameters are shown to be composite terms and are pressure dependent. Catalytically significant heats of adsorption have been extracted from the reaction data, and the TOFs have been found to be inversely proportional to the magnitude of the heat of adsorption of hydrogen.