Undigested dried sewage sludge from a wastewater treatment plant was pyrolyzed at temperatures between 300 and 900 degrees C, with an additional hold time at the highest temperature. A fixed-bed reactor was used with a heating rate of 20 degrees C/min under an atmosphere of nitrogen. Pyrolysis product distribution, FTIR, XRD, BET, SEM, and ultimate and proximate analyses were used to gain a better understanding of the structural changes occurring during pyrolysis. At low to medium pyrolysis temperatures, major mass loss occurs, and most of the gaseous and liquid products are released with little porous development, whereas at temperatures between 700 and 900 degrees C, structural changes seem to be triggered by calcium carbonate decomposition. T his leads to a second stage of gas evolution, as CaO promotes gasification of the char in the presence of iron sulfides. The subsequent release of CO runs parallel with an increase in the BET surface area. In addition, the aromatic character of the char increases with temperature, and nanotube-like tubular structures can be detected by SEM.