High-resolution electron energy loss spectroscopy has been used to study the coadsorption of CO and O on Ni(1 1 1) and Ni(1 0 0) surfaces at 250 K. These vibrational measurements showed that with increasing surface coverage at 250 K, the preadsorbed O caused changes in the adsorption sites for the after-adsorbed CO molecules revealing, however, dissimilar adsite alterations for the two Ni single crystals. Also, a different behavior towards carbonate formation was found upon CO adsorption at 250 K on Ni(1 0 0) and Ni(1 1 1) surfaces precovered with O at 250 K.