In this work we present an electrochemical and in situ scanning tunneling microscopy (STM) study of locally induced oxidation of the Cu(100) surface in naturally aerated 0.1 M NaClO4, pH <= 4 acidic solutions. The oxygen reduction reaction induces a near-electrode alkalization by the steady generation of OH-, producing a decaying pH gradient from the surface toward the bulk of the solution. The pH gradient profile and the concomitant surface processes are dependent upon the bulk solution pH, polarization potential, and polarization time. The local alkalization occurring in the presence of dissolved oxygen in the solution influences substantially the surface properties. These are manifested by distinctive changes in electrochemical behavior and illustrated by in situ STM images in solutions with different bulk pH values. A semiquantitative estimate suggests almost 6 units of pH increase near the Cu(100) electrode in 0.1 M NaClO4 solution with bulk pH 2 after 25 min of polarization at anodic potentials. The similarity between electrochemical and STM results obtained in naturally aerated solution with bulk pH 4 and those registered in alkaline solutions with pH 9 confirms the local alkalization effect. (c) 2007 The Electrochemical Society.