The dependence of the potential of zero charge (pzc) for Pt(111) surfaces in acidic aqueous solution having increasing densities of ordered monoatomic steps in the (111)-(110) and (111)-(100) zones is evaluated from CO "charge-displacement" measurements, with the objective of elucidating the influence of the electrochemical double layer on the large step-induced changes in surface potential known for the clean uncharged surfaces in ultrahigh vacuum (UHV). This experimental strategy, which involves evaluating the charge flowing at controlled potentials upon "quenching" the aqueous double layer with chemisorbed CO, yields pzc values referring to zero "total" electronic charge, E-pzc(t). The E-pzc(t) values in both 0.1 M HClO4 and 0.5 M H2SO4 electrolytes decrease noticeably (by ca, 0.15 V) upon increasing the (110) step density, N, whereas smaller effects are found for (100) steps. The location of the E-pzc(t) values within the so-called "hydrogen" region, however, complicates interpretation of the E-pzc(t)-N dependences due to the presence of faradaic charge associated with potential-dependent hydrogen adsorption. Procedures are outlined by which this influence upon E-pzc(t), can be removed, yielding approximate estimates of pzc values referring instead to zero " free" electronic charge, E-pzc(f), as a function of step density. The analysis followed involves extrapolating charge-potential data from higher potentials where hydrogen adsorption is essentially absent, or evaluating instead "potentials of constant (nonzero) free charge" in this "double-layer" region, achievable most readily with the data in 0.1 M HClO4. The resulting E-pzc(f)-N plots exhibit substantial negative slopes for dilute (110) The E-pzc(t) values in both 0.1 M HClO4 and values approaching ca. -0.7 V being obtained, although the stepped surfaces (N less than or equal to 10(7) cm(-1)), Delta E-pzc(f) dependence changes sign close to the densely stepped (110) limit. Significantly, the E-pzc(f)-N profile obtained for (110) steps is comparable to the corresponding work function (Phi(M)-N) behavior for the Pt(111)-(110) surfaces in UHV. This indicates that the remarkably (ca. 1 eV) lower ＇＇local" Phi(M) values for Pt steps relative to (110) terrace regions known to be present in the latter environment are not attenuated (or otherwise altered) greatly by the presence of the aqueous double layer. Interpretation is given in terms of current understanding of interfacial solvation effects on E-pzc(f), and some electrochemical implications are pointed out.