The distributions of the total mercury (T-Hg), methylmercury (MeHg), and ethylmercury (EtHg) concentrations in soil and their relationship to chemical composition of the soil and total organic carbon content (TOC, %) were investigated. Core samples were collected from hill slope on the right and left riverbanks of the Idrija River. Former smelting plant is located on the right bank. The T-Hg average in each of the core samples ranged from 0.25 to 1650 mg kg−1. The vertical T-Hg variations in the samples from the left bank showed no significant change with depth. Conversely, the T-Hg varied with depth, with the surface, or layers several centimeters from the surface, tending to show the highest values in the samples from the right bank. Since the right and left bank soils have different chemical compositions, different pathways of mercury delivery into soils were suggested. The MeHg and EtHg concentrations ranged from n.d. (not detected) to 444 μg kg−1 and n.d. to 17.4 μg kg−1, respectively. The vertical variations of MeHg and EtHg were similar to those of TOC, except for the near-surface layers containing TOC greater than 20%. These results suggest that the decomposition of organic matter is closely related to organic mercury formation. Display omitted •The distributions of T-Hg, MeHg, and EtHg near the Idrija Hg mine were investigated.•The Hg values were compared to the chemical composition, TOC, and T-N of the soil.•The first data of EtHg in soil around the mine is reported.•The decomposition of organic matter is closely related to organic Hg formation.•Hg circulation among soil, air and plant with change of chemical forms was suggested.