Fouling in vacuum tower furnaces has been a great challenge in refineries. At the elevated temperatures of these units, coke deposition is common. For sour oils, iron sulfide also accumulates in the carbonaceous deposits, arising from iron dissolved in the feed or from corrosion of metal surfaces. In this work, the combined fouling mechanism is investigated using an atmospheric tower bottom stream which contains 4.1wt.% sulfur, and 744 wppm dissolved iron. Isothermal deposition experiments running for 6-24hours have been carried out on a rotating cylinder in a 600-ml batch reactor, at bulk temperatures of 380-410 degrees C and cylinder rotation speeds of 300rpm. A ring of either carbon steel or alloy is mounted in the cylinder to serve as a corroding or noncorroding surface, respectively. After each experiment, the thickness and weight of deposit on the metal ring are determined. The compositions of deposit are characterized. The rates of deposit formation on the ring and the deposit composition are reported as functions of temperature for a number of metal surfaces. Radial profiles of deposit composition are presented. The probable fouling mechanisms are discussed, based on the present results.