This study describes the distribution of arsenic mobile species in the tailings of CuCoNiarsenide using the sequential extraction and determining the contents of arsenate (AsV) and arsenite (AsIII). The object of this study is the tailings ponds of the Tuvakobalt plant, which contains waste from the hydrometallurgical arsenide ore processing of the Khovu-Aksy deposit (Republic of Tuva, Russia). A procedure of sequential extraction for arsenic was applied, and it includes the extraction of the following forms: water-soluble, potentially water-soluble and exchangeable, easily sorbed on the surface of carbonates, associated with Fe/Mn oxides/hydroxides, associated with easily oxidized minerals, and accounted for by non-oxidized arsenic minerals. This procedure, which takes into account the peculiarities of the physical and chemical composition of the waste, was supplemented by the analytical determination of the arsenite and arsenate content by using the methods of inductively coupled plasma atomic emission spectrometry (ICP-AES) combined with the hydride generation technique (HG-ICP-AES). The content of the most mobile forms of arsenic, which are water-soluble, potentially water-soluble, and exchangeable species, is equal to 56% of the total arsenic content, 23% and 33% of which are arsenite and arsenate, respectively. Unlike arsenic, the mobile forms of metals have been determined in small quantities. The largest proportion of water-soluble and exchangeable forms is formed by Mg, Ca, and Sr at 11, 9.4, and 20%, respectively (residual and redeposited carbonates). The proportion of water-soluble forms of other metals (Cu, Zn, Co, and Ni) is < 1% or 0. The main part of the metals is adsorbed on the surface of Fe and Mn hydroxides, enclosed in easily and hardly oxidized minerals. In addition to geochemical studies, the presence of leaks from the tailing ponds into ground waters was determined by using electrical resistivity tomography. The data obtained indicate a high environmental hazard of tailings and the possibility of water-soluble and highly toxic arsenic compounds entering ground waters and aquifers.