ATR-FTIRAS measurements were conducted to investigate nature of water molecules co-adsorbed with CO oil highly dispersed PtRu alloy and Pt catalysts Supported oil carbon black in the temperature range between 23 1,C and 60 degrees C. Each catalyst was uniformly dispersed and fixed by Nafion (R) film of 0.0125 mu m thickness oil a chemically deposited gold film. Adsorption of CO Was conducted and monitored by ATR-FTIRAS for 30 min in 1% CO saturated 0.1 M HClO4 after stepping the potential from 1.2 V and 1.0 V to 0.05 V and Pt/C and PtRu/C, respectively. Similar atop and bridge bonded CO hands were observed oil both PtRu/C and Pt/C, but a smaller relative band intensity, bridge bonded vs. atop CO, was observed on PtRu/C compared to Pt/C. A distinct O-H stretching band was found around 3643 cm(-1) and 3630 cm(-1) on PtRu/C and Pt/C, respectively, upon CO adsorption. They are assigned to non-hydrogen bonded water molecules co-adsorbed with CO oil these catalysts. We found that the number of non-hydrogen bonded water molecules co-adsorbed with a given number of CO molecules decreases with increasing temperature and is higher on PtRu/C than Pt/C at each temperature. We interpret the higher ability of water co-adsorption at PtRu/C over Pt/C is due to stronger H2O-metal interactions oil the alloy surface. We present a model of the CO-H2O co-adsorbed layer based on the bilayer model of water oil metal surfaces. (c) 2008 Elsevier Ltd. All rights reserved.