Pt nanoparticles (Pt-n) and their agglomerates (Pt-n(ag)) were synthesized by chemical reduction methods. The mean sizes of Pt-n and Pt-n(ag) were determined to be respectively 3.4 and 400 nm using transmission electron microscopy (TEM). CO adsorption on electrodes made of the two kinds Pt nanomaterials was used as the probe reaction. Studies of cyclic voltammetry revealed that the current peak potential Of COad oxidation on electrodes of Pt-n(ag) supported on glassy carbon (Pt-n(ag)/GC) has been shifted negatively by about 80 mV in comparison with that on a bulk Pt electrode. In situ electrochemical FTIRS results illustrated that Pt-n/GC and Pt-n(ag)/GC exhibit different IR properties for CO adsorption. In comparison with CO adsorbed on bulk Pt, a phenomenon of enhanced IR absorption (EIRA) was observed on dispersed Pt nanoparticles (Pt-n). IR absorption of COad on Pt-n/GC is enhanced 31 times and the full width at half-maximum (FWHM) of the COL band is increased to 40 cm(-1), which is 25 cm(-1) larger than the value (15 cm(-1)) measured on a bulk Pt electrode. In contrast to the EIRA of CO adsorbed on dispersed Pt nanoparticles, the abnormal infrared effects (AIREs) were encountered for CO adsorbed on agglomerates of Pt nanoparticles, in which a strong interaction is presented between Pt-n. The IR features of CO adsorbed on Pt-n(ag)/GC illustrated that, except for the IR absorption of COad being enhanced 9 times and the FWHM of the COL band being increased to 25 cm(-1), the direction of the COad bands is inverted from absorption to antiabsorption. The results of the present paper demonstrated that the EIRA and the AIREs are closely related to the agglomerate states of Pt nanoparticles, and throw a light on the origin of the particular IR properties of nanomaterials.