The COR-TEN weathering steel is increasingly treated with rust stabilizers for use in maintenance-free structures. The effect of the rust stabilizer coating and the chemical structure of the stabilized rust film formed after atmospheric exposure are described. When exposed in the suburban atmosphere, the rust stabilizer-coated COR-TEN steel developed a stable rust film as originally expected and its corrosion was substantially inhibited. In the marine atmosphere, its sunlit surface exhibited corrosion behavior similar to that observed in a suburban area, while its sheltered surface failed to form a stable rust film even after several years of atmospheric exposure. Our conclusion is that the use of COR-TEN steel requires consideration given to both the atmospheric environment and its location in the structure.