IR optical properties of Pd nanoparticles with different size and aggregation state were studied in the current paper. The dispersed Pd nanoparticles (Pd-n(a)) stabilized with poly(N-vinylpyrrolidone) (PVP) were synthesized by the seeding growth method, in which the seeds were formed step by step through reducing H2PdCl4 with ethanol. The dispersed Pd nanoparticles of much large size (Pd-n(b)) were grown from the Pd-n(a) by keeping the colloid of Pd-n(a) undisturbed for 150 days at room temperature around 20 degrees C. The aggregates of Pd-n(a) (Pd-n(ag)) were prepared through an agglomeration process induced during a potential cyclic scanning between -0.25 V and 1.25 V for 20 min at a scan rate of 50mVs(-1). Scanning electron microscope (SEM) patterns confirmed such aggregation of Pd-n(a). Fourier transform infrared (FTIR) spectroscopy together with CO adsorption as probe reaction was employed in studies of IR optical properties of the prepared Pd nanoparticles. The results demonstrated that CO adsorbed on Pd-n(a) films substrated on CaF2 IR window or glassy carbon (GC) electrode yielded two strong IR absorption bands around 1970 cm(-1) and 1910 cm(-1), which were assigned to IR absorption of CO bonded on asymmetric and symmetric bridge sites, respectively. Similar IR bands were observed in spectra of CO adsorbed on Pd-n(b) films, except the IR bands were much weak, whereas CO adsorbed on Pd-n(ag) film produced an IR absorption band near 1906cm(-1), and an anomalous IR absorption band whose direction has been completely inverted around 4956 cm(-1). The direction inversion of the IR band of CO bonded to asymmetric bridge sites on Pd-n(ag) was ascribed to the interaction between Pd nanoparticles inside the aggregates. Based on FTIR spectroscopic and cyclic voltammetric results, the aggregation mechanism of Pd nanoparticles from Pd-n(a) to Pd-n(ag) has been suggested that the agglomeration of Pd nanoparticles was driven by the alteration of electric field across electrode-electrolyte interface, when the PVP stabilizer was stripped via oxidation during cyclic voltammetry. (c) 2005 Elsevier Ltd. All rights reserved.