The highly toxic element, arsenic, creates worldwide health problem. In Sweden, arsenic is the single most common contaminant among the country´s worse risk classified sites. Billions of Swedish crowns will therefore in future be used for site remediation measures. To prevent increasing remediation costs, cost-efficient methods are highly favorable.Fly ash is a waste product arriving from combustion of fuel. Landfilling of fly ash from municipal solid waste incineration is today restricted in Sweden and since 2005 tons of the material are exported to Norway. New field of application for the material is needed within Sweden. Fly ash has properties that effects arsenic and could be useful for remediation techniques. The aim of this report is to investigate the mobilization effects fly ash has on arsenic and if the effects could be enhanced and used for arsenic remediation. The research is based on two practical experiments and a literature review about the chemistry of arsenic.The aim of the first experiment was to immobilize arsenic within soil by utilization of fly ash. The tests were performed with addition of ash, ash + peat or zerovalent iron to the soil samples. Soil pore water sampling and analysis were performed after 2, 4 and 7 weeks.In the second experiment, soil containing arsenic was washed with water and ash, for the purpose of washing arsenic out from the soil. The experiment was performed with addition of 0%, 10% or 20% ash at the liquid to solid (L/S) ratios of 2, 10 and 100. The washing at L/S 2 was also performed at two different temperatures of 60°C and 28°C.The results from the stabilization test, where the aim was to immobilize the arsenic, showed that fly ash highly mobilizes the element. The soil pore water from the tests containing ash had higher concentration of arsenic, dissolved species and increasing pH. The arsenic mobilization was probably due to the elevated pH, which increases the mobility of especially the pentavalent arsenic, arsenate. The results from addition of peat showed a small decline of arsenic within the pore water. The addition of fly ash did therefore not stabilize the arsenic within the soil, but mobilized the element and increased the concentration in the soil pore water during the whole 7 weeks of experiment.The soil wash, however, showed that fly ash also have an immobilizing effect on arsenic. The leachate from the samples where ash was used contained lower arsenic concentrations than the leachate from samples without any ash addition. The inhibition of arsenic release could be explained by formation of precipitating products especially between arsenic and manganese and calcium. Due to the addition of ash the available surface sites for arsenic adsorption did also increase, which might have contributed to the immobilization of the element. The highest arsenic concentration was found in the leachate from the highest L/S ratio, L/S 100. The samples containing ash that were washed at the higher temperature of 60°C had lower arsenic concentrations in the leachate. The cost-efficient amendment, fly ash, would be beneficial for soil remediation. However, the effects ash causes to arsenic and its surrounding environment require further research.