The hydrogenation of phenyl acetylene and styrene have been studied over a palladium/carbon catalyst. The kinetics of the reactions were investigated and activation energies of 26 +/- 2 kJ mol(-1) and 41 +/- 8 kJ mol(-1) were obtained for phenyl acetylene hydrogenation and styrene hydrogenation, respectively. Both reactions were found to be zero order concerning the alkyne or alkene. However the order in phenyl acetylene changed from zero order to first order at approximately 60% conversion. This change was due to the effect of styrene co-adsorption and not the concentration of phenyl acetylene. Competitive hydrogenation between the alkene and alkyne resulted in a dramatically reduced rate of hydrogenation for both species. This reduced rate was explained by a reduction in the amount of surface hydrogen as well as the altered bonding of the phenyl acetylene.