The effects of pretreatment of catalyst on its surface properties and the HDS activity of a 0.49% Ru/AI(2)O(3) catalyst were studied in a single-pass, differential microreactor. The surface properties of the catalyst were measured by NH3-TPD and XPS analysis. The Ru/Al2O3 catalyst was pretreated in three ways : reduced in H-2 (Ru-R catalyst), oxidized in air and subsequently reduced in H-2 (Ru-OR catalyst), or sulfided in H2S/H-2 (Ru-S catalyst). Three types of peaks (low, middle, and high temperatures) were observed in the NH3-TPD study. The predominant high-temperature peak was observed for both the Ru-OR and Ru-S catalysts, pretreated at 300 degrees C. Mass spectrometry showed that the high-temperature peak in NH3-TPD consisted of N-2 and H-2 formed from the decomposition of NH3 on the ruthenium sites. NO adsorption of unsaturated Ru species was related to the low-temperature peak in the NH3-TPD. The XPS analysis showed that the peaks at 279.9 eV, 280.6 eV, and 282.5 eV were ascribed to metallic ruthenium, RuO2, and RuO3, respectively. The low-, middle-, and high-temperature peaks were assigned to RuO2, acid sites on alumina, and metallic Ru, respectively. Metallic ruthenium was effective in the HDS of thiophene and the decomposition of NH3.