The main aim of this study is to evaluate the stabilization of As-contaminated soil with Fly Ash (FA) and the environmental impact assessment of a soil/ash mixture after treatment. Two soil samples heavily contaminated with As were stabilized with two FA that originate from incineration of biofuels (BFA) and mixture of coal and crushed olive kernel (CFA). The As solubility was reduced between 39 and 93% in soil/ash mixtures. Leaching of As from soils decreased with increased amount of ash. Immobilization of As was positively correlated to pH and negatively correlated to the amount of organic matter in soil. Geochemical modeling showed that the release of As was controlled by Ca3(AsO4)2:4H2O in soil/ash mixtures that can be a possible explanation for the reduced solubility of As from soil/ash mixtures. A negative effect of the treatment was the mobilization of Cu from ashes and Dissolved Organic Carbon (DOC) from soil.