Alumina and zirconia supported Pt and Pt-Sn catalysts have been tested in the dehydrocyclization (DHC) of n-octane at 823 K to alkylaromatics as desired products using pure hydrogen (H-2) and water vapour containing hydrogen (WVH2) atmospheres. The materials were characterized by nitrogen adsorption, NH3-TPD, TPR, XRD, XPS, TEM and EDX The characterization results reveal the formation of PtSn alloys on alumina support after a catalyst precursor reduction in H-2 at 873 K. This observation enables an interpretation of the higher on-stream stability of Pt-Sn/Al2O3 catalyst in H-2 as well as in WVH2 Compared to the monometallic sample, using the well-known concept of the ensemble effect of Sn by alloying to Pt. The alumina supported Pt and Pt-Sn catalysts act bifunctionally in H-2 atmosphere, producing a mixture of benzene, mono- and dialkyl aromatics, near the chemical equilibrium. In WVH2 atmosphere, however, o-xylene and ethylbenzene are predominant among the aromatic products, indicating a monofunctional action mode of the catalyst. Independent of the reaction atmosphere, the zirconia supported Pt and Pt-Sn catalysts act monofunctionally with o-xylene and ethylbenzene as predominating products among the aromatics produced. The zirconia supported samples are more stable in WVH2 than in H-2 atmosphere. The addition of Sn significantly increases the activity of that catalyst but does not retard its deactivation in H-2. The formation of PtSn alloys is not evidenced in the Pt-Sn/ZrO2 catalyst after hydrogen treatment. The behaviour of the zirconia supported catalysts can be interpreted considering the weak-acidic character of the support and its interactions with Pt as well as with SnO2. (c) 2007 Elsevier B.V. All rights reserved.