In many previous studies, nonaqueous synthesis of Pt nanocubes with tunable size has been achieved by the use of metal carbonyls (e.g., Fe(CO)(5), Co(2)(CO)(8), W(CO)(6)). The presence of zero-valent metals in the carbonyls was demonstrated as the key factor to the nanocube formation but the role of CO was entirely ignored. By using CO alone, we have now demonstrated that the favorable growth of Pt nanocubes in the presence of CO is mainly owing to the effect that the Pt (100) surface is stabilized by the co-adsorption of CO and amine.