A detrimental effect of CO on the stability of Pt nanoparticles, which is important for fuel cells technology, is demonstrated by the change in HCOOH kinetics on Pt nanoparticles caused by the presence of CO in the electrolyte. Pt nanoparticles supported on high area carbon (Pt/C) were applied on gold and glassy carbon substrates (Pt/C@Au and Pt/C@GC) and three different kinds of Pt surface were prepared by potential cycling in the various potential limits in N-2 and CO saturated electrolyte: untreated, OH-annealed and CO-annealed. On untreated Pt/C@Au catalyst HCOOH is oxidized predominantly through indirect mechanism, after OH-annealing direct path is slightly accelerated, but on CO-annealed catalyst the direct path in HCOOH oxidation is predominant. Based on the fact that more contiguous Pt sites are necessary for indirect than for direct path, it was concluded that OH- and especially CO-annealing of Pt/C produces small Pt domains. Since the same pretreatment did not show any effect on HCOOH oxidation on Pt/C@GC electrode, it was postulated that Pt nanoparticles dissolve in the presence of CO on the surface and in the electrolyte, and successively deposit on Au substrate. The results points out pronounced instability of Pt nanoparticles in the presence of CO. (C) 2011 Elsevier B.V. All rights reserved.