In the framework of a two stage process for the hydrotreatment of gas oil, the use of noble metal catalysts in the second stage, where the H2S partial pressure is considerably decreased, was proposed. Thus, the catalytic properties of a series of PtxPd100-x (x corresponding to at.% in the alloy) particles supported on zirconia were investigated in tetralin hydrogenation and decahydroquinoline hydrodenitrogenation. TEM micrographs and EDS analysis evidenced the high dispersion of the monometallic and bimetallic composition of the nanoparticles. Whereas a promoting effect is observed in the case of hydrogenation reaction, the introduction of Pd suppressed the catalytic activity in C-N bond scission reaction. This poisoning effect can be simply understood as a linear effect of the segregation of Pd at the surface of PdPt particles. The effect of H2S partial pressure on the HDN activity was investigated and showed that for this reaction thioresistance is not improved by alloying. (C) 2007 Elsevier B.V. All rights reserved.