The behavior of a Ru/Al2O3 catalyst for the preferential CO oxidation in the presence of low-concentration NH3 (21-75 ppm) was investigated. In contrast to the results in the absence of NH3, the catalyst was rapidly deactivated at higher [O-2]/ [CO] ratios in the presence of NH3. NH3 was oxidized to nitrosyl species, which was strongly adsorbed on Ru and suppressed the activation of both oxygen and CO. At low [O-2]/CO] ratios, the catalyst exhibited a stable activity even in the presence of NH3, and the deactivated catalyst was rapidly regenerated.