Fourier transform infrared (FTIR) spectra are presented of CO gas-treated protected gold colloids prepared from hydrazinium hydrate reduction of an Au(III) precursor which reproducibly feature a weak, shortlived peak at ca. 2169 cm(-1). When the gold colloid was treated with 99% isotopically enriched (CO)-C-13 gas, the IR peak shifted to a frequency of 2114 cm(-1) which indicated that it represented a simple gold monocarbonyl species. The value of 2169 cm(-1) for the CO stretching frequency suggests the peak represents CO physisorbed on oxidised gold atoms on the colloid surface. The peak is not observed when the concentration of the colloidally dispersed gold is reduced either by use of lower starting salt concentrations or by aggregation. It is also not observed when solutions of the protecting agent or reducing agent or the dispersion medium (water) or even the starting Au(III) salts are CO-treated individually. This confirms that the spectral feature is uniquely associated with colloidally dispersed gold. In general, the work has shown that the surfaces of Au colloids in situ have partially oxidised An character which is of interest in systems where supported nanoparticulate gold derived from colloid preparations are considered for low temperature oxidation catalysts for CO. (c) 2007 Elsevier Inc. All rights reserved.