Deactivation of a Pd/alumina catalyst has been observed during the hydrogenation of α-methylstyrene and styrene. In both feedstocks, deactivation is caused by an additive, 4-tert-butylcatechol (TBC), a polymerization inhibitor, commonly employed at the ppm concentration level in the formulation of commercial monomers. It was found that the reaction rate in the α-methylstyrene fluctuated notably among the reactant vendors, and this was ascribed to the varying concentration of TBC, although other factors, such as the concentration of water, may play a role. The study was extended into the hydrogenation of styrene using a trickle bed reactor. The negative impact of the TBC present at the ppm level was obvious. The deactivation mechanism was complex to rationalize. A two-stage behavior was observed: a first stage of a relatively fast deactivation followed by a second stage of slow deactivation. A tentative explanation considers the presence of two types of Pd-sites, which are poisoned by TBC: the more active α-Pd-H sites and the less active β-Pd-H sites. Finally, in practical terms, it is important to emphasize that such an additive must be removed from the reactant to maximize the catalyst performance. This can be achieved by adsorption using a commercial F-200 Alcoa alumina.