Results of testing to determine the physical properties and chemical leaching characteristics of five different flue gas cleaning (FGC) sludges and the stabilized/solidified products from five commercially available treatment processes are reported. This study suggests that solidification/stabilization of FGC wastes may be a feasible method of reducing their pollutant potential in landfilling. However, a great deal more study is necessary before the behavior of treated FGC sludges under actual field conditions can be adequately understood.