Electrode potential-dependent infrared spectra for carbon monoxide dosed onto carbon-supported platinum nanoparticle films, significant as commercial fuel-cell catalysts as well as of fundamental importance, are reported with the aim of elucidating their structure as a function of particle size. The kneed to acquire absolute unipolar, rather than bipolar, spectra by means of potential-difference infrared tactics for such nanoparticle films is demonstrated, given the broad asymmetric C-O stretching band shapes. For larger particle diameters (d greater than or equal to 4 nm), the potential-dependent peak stretching frequencies (v(CO)(P)) for saturated CO are closely similar to atop CO on Pt(111) electrodes, indicating a preponderance of 9-coordinate Pt sites. However, for nanoparticle diameters in the range d approximate to 2-4 nm, the v(CO)(P) values at a given potential, E, redshift sharply with decreasing d, approaching frequencies compatible with those measured at the same surface potential for atop CO in chargeable high-nuclearity Pt carbonyl solutes. The latter, structurally well-characterized, nanoparticles are known to contain predominantly edge- rather than (111) terrace-bound CO. The implication that the nanoparticle size-dependent structural transition is associated with changes in the Pt surface coordination number, consistent with pseudo-spherical packing-density considerations, is supported by comparisons of the v(CO)(P)-E data for co lower CO coverages with corresponding potential-dependent spectra for CO bound to step sites on high-index Pt electrodes. The broad-based value of vibrational measurements at controlled surface potentials for characterizing conducting nanomaterials is pointed out.