The effect of LaCl3 on the properties and deactivation of Pd/SiO2 catalysts by thiophene poisoning in the hydrogenation of ethylbenzene has been studied. Gas chemisorption and TPR measurements, respectively, show that Pd dispersion increases with the ratio La/Pd and that La hinders palladium reduction. The presence of Pd-n+ species on Pd/SiO2 was evidenced by XPS, but the incorporation of lanthanum decreases the concentration of Pd-n+. Deactivation results show that La improves the Pd sulfur resistance, which goes through a maximum for the catalyst with La/Pd = 2 atomic ratio. This effect cannot be attributed to the increase in Pd dispersion produced by La, a decorating effect of Pd by La species, or sulfur adsorption by La. From the mechanism for metal poisoning by thiophene at 493 K, which involves the hydrogenolysis of the sulfur compound on the metal sites, an alternative explanation of the results is suggested. It seems that LaCl3 essentially plays a diluent role, disrupting the ensembles of palladium atoms required for the breaking of the C-S bond. XPS and TPR results, which indicate the occurrence of a weak Pd-La interaction, support this hypothesis. Thus, the catalysts containing lanthanum are more sulfur resistant, with a maximum for La/Pd = 2. For this sample, XPS suggests that a slight charge transfer from Pd to La takes place and therefore the electronic effect of Pd-n+ is added to the geometric effect produced by La3+.