Infrared reflection-absorption spectra (IRAS) measured for the C-O stretch (nu(CO)) of carbon monoxide dosed on ordered Pd(100) in aqueous 0.1 M HClO4 as a function of the coverage (theta(CO)) and the electrode potential (E) are examined in comparison with extant IRAS and adlayer structural data on Pd(100) in ultrahigh vacuum (UHV). A noteworthy feature of the electrochemical adlayer, as for the UHV system, is the presence of a single vco band blue shifting markedly (by ca. 100 cm(-1)) with increasing coverage up to saturation (theta(CO) approximate to 0.8), indicative of uniformly bridging CO coordination. Extrapolation of the theta(CO)-dependent nu(CO) frequencies to surface potentials corresponding to the UHV-based system, however, yielded reasonable concordance only at high coverages. The discrepancies at lower theta(CO) are suggested to arise from solvation effects on the local GO-site surface potentials. Examination of the theta(CO)-dependent vco peak frequencies (nu(CO)(P)), integrated absorbances (A(i)), and bandwidth (A nu(1/2)) for the dosed electrochemical adlayers reveals marked alterations at theta(CO) approximate to 0.5. The unusual nonmonotonic form of the A(i) - theta(CO) and Delta nu(1/2) - theta(CO) dependencies are discussed in relation to the behavior observed for the UHV-based system. The observed similarities suggest the occurrence of a phase transition for the electrochemical system as in the well-characterized UHV case, involving the formation of phase/antiphase adlayer domains for theta(CO) > 0.5. The relation between the nu(CO)(P) - theta(CO) behavior observed for adlayers formed by dilute CO solution dosing and by progressive electrooxidative removal from an initially saturated layer is also considered. Unusually, the hystersis observed between these plots is only minor, and restricted to intermediate coverages (theta(CO) similar to 0.4-0.6). This behavior indicates that the CO domains formed upon partial adlayer electrooxidation are small and/or spontaneously dispersed. The possible connection between metal surface oxidation and CO adlayer oxidation on Pd(100) is considered in the light of the IRAS and voltammetric data.