The influence of the structure of bimetallic platinum ruthenium electrocatalysts towards the electrooxidation of methanol has been investigated by cyclic voltammetry and coupled electrochemical-in situ infrared reflectance spectroscopic (FTIRS) measurements. Three PtRu electrocatalysts with different structures were prepared via the colloidal route, either by coreduction of the metal salts or by mixing the metal colloids. In each case, the bulk atomic composition of the catalysts was kept constant, i.e. 80:20 Pt/Ru. Characterisation of the catalyst particles by transmission electron microscopy and nano-energy dispersive X-ray indicated that the size distribution was around 2 nm, confirmed the bulk atomic composition and showed alloy characteristics for the coreduced catalyst, whereas the non-coreduced catalysts were composed of platinum and ruthenium particles. The activity of these electrocatalysts dispersed onto high surface area carbon (Vulcan XC72) was determined by CO stripping and towards the electrooxidation of methanol, showing that the alloyed compound displayed the lowest activity compared with the other catalysts. The results obtained were interpreted in terms of a better ability to dehydrogenate methanol and an increased CO tolerance in the case of non-alloyed catalysts compared with the alloyed ones. (C) 2003 Elsevier B.V. All rights reserved.