Fly ash from a cogeneration plant near Sundsvall in Sweden was treated in an ash-washing facility. The leaching of chromium (Cr) and molybdenum (Mo) from the ash residue exceeded the limit values for non-hazardous landfills. In this study factors that influence the leaching of Cr and Mo were identified and methods that can reduce the leaching were evaluated. The results revealed that the mobility of Cr and Mo are mainly controlled by pH and redox reactions and sequential extraction tests also showed that the fraction of highly soluble species of Cr and Mo increased after washing due to pH reactions in the ash during the process. Stabilization of the pH at ∼8 through carbonation of the washed ash and a lowering of the redox potential by adding ferrous iron to the process resulted in decreased leaching. Treatment with carbon dioxide yielded a decrease (from 10.7 to 8.2) in the pH and, hence, the leaching of Cr and Mo by 93 and 91%, respectively. And the addition of ferrous iron reduced the leaching of Cr by 50%. Carbonation of the ash can be achieved via treatment with flue gases from the power plant or treatment with landfill gas at the disposal site.