In-situ infrared reflection-absorption spectra are reported for mixed nitric-oxide-carbon-monoxide adlayers along with the constituent chemisorbates separately as a function of coverage on ordered Ir(111) at 0.4-0.45 V vs standard hydrogen electrode in aqueous 0.1 M HClO4, with the objective of assessing the composition-dependent nature of the coadsorbate vibrational interactions. This substrate-coadsorbate combination provides an informative model system since both chemisorbates appear to bind exclusively in atop (or near-atop) surface sites on the basis of their simple N-O (nu(NO)) and C-O (nu(CO)) vibrational fingerprints, and composition-dependent mixed adlayers can readily be formed via partial replacement of saturated irreversible adsorbed NO layers by exposure to dilute CO solutions. Increasing the CO coverage, theta(CO), both in the absence and presence of coadsorbed NO, yields marked progressive blueshifts in the vco band frequency (ca. 2020-2075 cm(-1)), attributable chiefly to enhanced dipole-dipole coupling. While adsorption of NO alone exhibited virtually coverage (theta(NO))-independent nu(NO) frequencies, ca. 1835 cm(-1), indicative of chemisorbate island formation, dilution within mixed CO/NO adlayers yields progressive nu(NO) redshifts (down to ca. 1790 cm(-1)). The composition-dependent nu(CO) and nu(NO) frequencies within the CO/NO adlayers are consistent with molecular intermixing, as supported by comparison with numerical simulations extracted from conventional dipole-coupling theory, although the observed nonlinear nu(CO)-theta(CO) dependence suggests the formation of locally enriched CO regions at intermediate compositions. Evidence supporting coadsorbate intermixing is obtained by comparing the composition-dependent CO and NO band absorbances with the dipole-coupling predictions. In particular, the presence of coabsorbed CO yields marked (up to 3-fold) decreases in the NO band absorbance, especially toward lower nu(NO) values, which arise from band-intensity transfer to neighboring higher-frequency (CO) oscillators. Despite the large (ca. 250 cm(-1)) difference in nu(NO) and nu(CO) singleton frequencies,this striking effect is in approximate agreement with dipole-coupling theory, again presuming molecular CO/NO intermixing. The observed marked increases in the vco bandwidth toward lower theta(CO) values, along with pronounced asymmetric band shapes, are in good agreement with theoretical predictions that include stochastic fluctuations of the local adsorbate population density, Moreover, the larger intermediate-theta(CO) nu(CO) bandwidths observed in the presence of coadsorbed NO are also quantitatively accounted for on this basis in terms of coadsorbate intensity transfer. The more broad-based utility of such dipole-coupling analyses for elucidating local interactions within mixed adlayers is considered in light of these findings.