Use of the syngas produced by biomass gasification is rendered unviable because of impurity compounds, whose efficient removal is key to the feasibility of gasification concepts. Tars present in the biomass gasification gas can be efficiently oxidized by ZrO2 catalysts. H2S has been shown to increase the oxidation activity on ZrO2 and most notably on La2O3-ZrO2. Gasification gas clean-up experiments were performed at 600–800 °C to study the beneficial effect of H2S on naphthalene and toluene conversions. During time-on-stream with repeated doses of H2S on ZrO2, cumulative and strong H2S adsorption was observed as the conversion of toluene and naphthalene increased during the first H2S doses and stabilized towards the end. Therefore, a limited capacity of the catalyst to adsorb H2S was suggested, which is the origin of sulfur tolerance. Activity enhancement with H2S occurs most notably on La2O3-ZrO2 and towards naphthalene oxidation. XPS characterization of the used catalysts showed sulfur retention on the La2O3-ZrO2 but not on pure ZrO2, indicating that La2O3-ZrO2 holds more sulfur on its surface or binds it stronger. The surface properties of La2O3-ZrO2 were thus characterized with in situ DRIFTS using CO2 and CH3OH as probe molecules. The surface of La2O3-ZrO2 was suggested to have more low coordination sites than ZrO2, where sulfur is suggested to be bound and alter the reactivity of the surface oxygen.