The sites of historical arsenic mining and processing are often considerably polluted with As and other potentially toxic elements, which poses a considerable environmental risk. Organic matter supplied to those soils as decomposing forest litter can cause mobilization of As and other toxic elements, change their speciation in pore water and influence the toxicity to biota. This study examined the chemistry and ecotoxicity of pore water acquired from four soils that developed on the dumps in former As mining sites, in the presence and absence of organic matter introduced with forest litter collected from beech and spruce stands. Soils that contained 1540-19600 mg/kg of As were incubated for 90 days at 70% of water holding capacity. Soil pore water was collected four times (after 2, 7, 21 and 90 days) with MacroRhizon suction samplers and analysed. Chemical analysis involved determination of pH, concentrations of As and metals that had a high enrichment factor (Igeo>3): Cu and Pb, as well as other metals considered most mobile in the environment: Cd, Zn and Mn. Ecotoxicity of pore water was examined in three bioassays: Microtox, MARA and Phytotox with Sinapis alba as test plant. The release of As, unlike heavy metals, was particularly intensive from the soils with neutral and alkaline pH. The results provided by Phytotox had a poor precision, but correlated well with As concentrations in pore water, which indicates that As made a crucial factor of phytotoxic impact on the seedlings. Similarly, the outcomes of Microtox bioassay also indicated a relationship between As concentrations and toxicity, however, other factors contributed to ecotoxicity at very low and very high As concentrations, and the highest toxicity was recorded from the soils treated with forest litter. MARA turned out to be not sensitive enough to give reproducible results at those concentrations of toxic elements that were present in pore water, up to dozens mg/L. The growth of microbes in the MARA bioassay was poorly dependent on the concentrations of As and metals in pore water except for a yeast Pichia anomala the growth of which was related to the concentrations of Cu. The results let us conclude that the bioassays Phytotox and Microtox can provide useful information on ecotoxicity of pore water in soils that develop on As-rich dumps whereas applicability of MARA in those conditions proved limited.