Model dipole-dipole coupling calculations of infrared spectra for various ordered CO packing arrangements on Pt(III) have been undertaken as a function of the spatial dimensions of isolated nanoscale adlayer domains. Comparisons of some of these predictions with coverage (theta(CO))-dependent infrared spectra obtained upon partial adlayer oxidation on low-index Pt-group surfaces in aqueous electrochemical environments enable approximate estimates of densely packed CO island sizes to be obtained. The evidence for CO island formation, described in several earlier reports, is based on the observation of C-O stretching bands, nu(CO), whose frequency, band shape, and relative intensity exhibit markedly smaller alterations upon progressive electrooxidative removal in comparison with corresponding nu(CO) frequency-coverage data obtained for increasing theta(CO) by means of dosing with dilute CO solutions. The dipole-coupling calculations indicate a significant sensitivity of the nu(CO) frequency and related parameters to island size even for domains containing sizable numbers (100-200) of CO molecules, suggesting the value of the procedure for providing at least rough estimates of nanoscale island dimensions. While the domain size-dependent nu(CO) spectra should be dependent inevitably upon the microscopic CO packing geometry, the (relative) changes in the vco frequency for atop (i.e. terminal) adsorbate are predicted to be dependent on the island dimensions, yet relatively insensitive to the adlayer structure, suggesting the practical utility of this parameter for island size diagnosis. The resulting estimates of average island dimensions, along with their stability as gleaned from experimental time-dependent spectra, are seen to vary widely on different low-index Pt-group electrodes. Interestingly, the metastable CO domains formed on Pt(100), estimated to contain 20-50 atop CO＇s, have dimensions roughly compatible with the restructured substrate "mesas" seen to form by scanning tunneling microscopy.