The friction and wear characteristics of automotive friction materials are strongly dependent on the composition and microstructure of the rotor surface. In this study we investigated the compositional and microstructural changes occurring in the surface layers of cast iron brake rotors during dynamometer tests with a typical organic friction material. Rotors were studied in the as-manufactured, lightly ground and sanded, and as-burnished conditions, as well as after 30 stops from 60 mph at a deceleration rate of 15 ft/s2. Optical and scanning electron microscopes were used to examine the surfaces. Minimum disturbance of the microstructure was found in the sanded surface, but the as-manufactured and burnished surfaces exhibited considerable disturbance.After the 30 stops the pearlite was transformed locally into martensite. Composition analysis of the burnished rotor surface showed high magnesium content. This was interpreted as asbestos or olivine from the lining, transferred to and embedded in the drum surface. These findings indicate that cast iron rotors in brake systems are not, as commonly envisioned, passive, relatively unchanging parts of the friction couple (as compared with the friction material). Instead, they are metallurgically quite active. For this reason they must be well characterized during tests of friction materials.