Many alloys possess a heterogeneous surface exhibiting a varying electron transfer rate across the surface. For example, the aluminum alloy AA 2024-T3 consists of Cu-containing intermetallic inclusions that are rather sparsely distributed within the Al alloy matrix. The electron transfer rate on the matrix and at the Cu sites can be estimated from probe approach curves using scanning electrochemical microscopy (SECM). Such information is useful for understanding corrosion of the alloy and/or electrodeposition (e. g., of polymer coatings) on the alloy. While measurement of the electron transfer rate on the matrix is straightforward, measurement at the Cu sites is problematic since these sites are small (on the micron scale) and are surrounded by matrix at which electron transfer may also occur, a situation that has not been modeled previously. In this work we use COMSOL Multiphysics software to generate approach curves for an irreversible electron transfer reaction at a site of small dimension surrounded by a matrix that exhibits slower (or faster) electron transfer rate. Of particular interest are the influences of site size and the ratio of the rate constants (at the site and matrix) on the approach curves. Initial attempts at fitting experimental data with this alloy model are also described. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.034207jes] All rights reserved.