Abstract: The migration and release behaviors of thallium in the deposed sulfuric acid slag, collected from a sulfuric acid plant in Guangdong, in the process of natural leaching were intensively investigated. It indicates that thallium of slag has distinct release ability in the process of natural leaching, thallium speciation is an inherent and chief factor, and the pH conditions of slag and its mineral composition are the exterior and subordinate factors. The release ability of thallium bound to different minerals phase is in the following order: acid exchangeable fraction > bound to Fe/Mn oxide (hydroxide( > bound to sulfide and silicate minerals.