The effect of the support (activated carbon or titanium dioxide) on the catalytic activity and selectivity to nitrogen of Pt-Sn catalysts in nitrate reduction was studied. The effects of the preparation conditions and the Pt:Sn atomic ratio were also evaluated. It was observed that the support plays an important role in nitrate reduction and that different preparation conditions lead to different catalytic activities and selectivities. Generally, the catalysts supported on activated carbon were less active but more selective to nitrogen than those supported on titanium dioxide. The monometallic Pt catalyst is active for nitrate reduction only when supported on titanium dioxide, which is explained by the involvement of the support in the reaction mechanism. The catalysts were characterized by different techniques, and significant changes on metal chemical states were observed for the different preparation conditions used. Only metallic Pt and oxidized Sn were observed at low calcination and reduction temperatures, but some metallic Sn was also present when high temperatures were used, being also possible the formation of Pt-Sn alloys. (C) 2011 Elsevier Inc. All rights reserved.