The coordination of CO at the surface of very small ruthenium nanoparticles has been investigated by IR and solid state NMR spectroscopies. Two sets of ruthenium nanoparticles stabilized either by a polymer (polyvinylpyrrolidone; PVP) or a ligand (bisdiphenylphosphinobutane; dppb) have been studied in order to evidence any influence of the stabilizer on the location and dynamics of CO molecules at the particles surface. It was found that CO groups are mobile on the surface of the nanoparticles even in the solid state and that bulky ancillary ligands such as dppb may slow down the fluxionality of CO and prevent exchange at certain positions.