In this study, the chloride migration coefficient of cement-based materials was obtained by using the accelerated chloride migration test (ACMT) and measured as a function of volume fraction of aggregate and the total lateral surface area of aggregate. In order to study the effect of the increased migration coefficient on the ITZ and the decreased migration coefficient in the matrix as aggregate is added, two series of experiments (fixed volume fraction with varying total lateral surface area of aggregate, and fixed total lateral surface area varying volume fraction of aggregate) were performed in this study. A model considering the specimen as a three-phase composite (matrix, aggregate, and percolated ITZ) is used to determine the approximate chloride migration of the percolated ITZ and the inference of the matrix phase of mortar, as the rod-shaped aggregate is added. Based on the experimental and regression analytical results, the approximate percolated ITZ migration coefficient is 91, 97, and 109 times that of the matrix for the w/c ratio of 0.35, 0.45, and 0.55, respectively. (c) 2004 Elsevier B.V. All rights reserved.