The Idrija mine in Slovenia was the second largest mercury deposit in the world. Although the production in the mine stopped in 1995, the soil in surrounding of Idrija contains from 10 to even 10,000 mg Hg/kg at ancient roasting sites. Since Idrija is karstic area, surface and rain waters can leach Hg from contaminated soil to deeper soil layers and then into groundwater. In order to estimate the potential leaching of Hg, the experiments were conducted on contaminated soil sample containing 1347 mg Hg/kg. The leaching experiments have been performed in ultrapure water of different pH values in the range from 1.75 to 13.01, using standard batch method according to DIN 38414 S4. The concentration of total Hg in leachates obtained for pH range from 3.16 to 10.97 was very low. The highest concentration of mercury has been obtained in leachates in highly acidic (pH o = 1.75) and in highly alkaline (pH o = 13.01) range, which is most likely caused by the destruction of the organo-mercury complexes and consequently the release of mercury. Namely, acids and alkalis release Hg from the soil by dissolving or destructing the components in the soil that strongly attracts Hg. The complexation of Hg with organic matter (mostly humic and fulvic acids) is considered to be the most dominant processes controlling its mobility in soil. The increased concentration of leached Hg in alkaline conditions is most likely a result of the dissolving of humic acids. In acidic conditions it can be attributed to the dissolution of fulvic acids as well as organo-mineral complexes created by adsorption of dissolved mercury (which carries the humic and fulvic acids) onto mineral components in the soil. The share of leached Hg compared to its content in the soil was very small (from 0.0001% to 1.84%), even in highly acidic and the alkaline range, indicating that Hg was firmly bound to the soil matrix and was hardly leached by water. Nevertheless, even the smallest detected concentration of leached Hg exceed the maximum permissible levels for drinking waters according to Croatian and Slovenian legislations, as well as to the Council Directive of European Community, indicating the necessity of further monitoring the soil quality, as well as studying the Hg leachability at this contaminated sites of Idrija district.