Abstract <p>The formation conditions and existence borders for ⁶³Ni ionic and nonionic species in aqueous solutions containing anions with different complexing abilities (NO₃^–, NH₄⁺, SO₄^2–, C₂O₄^2–, EDTA, boric acid) were determined using ultrafiltration, centrifugation, and pH-metric titration. ⁶³Ni forms pseudocolloids and Ni(OH)⁺ hydroxo complexes in a medium of weak complexing ions at pH &gt; 6. The behavior of ⁶³Ni in the electrolyte solution for electrolyte–plasma treatment method is similar to its behavior in H₂SO₄ solution: At pH 8.0, ⁶³Ni(II) forms ammonium sulfate complexes that are retained by the membrane but do not undergo sedimentation during centrifugation. The presence of hydrolyzed Fe(III) species in solution leads to an increase in the retention of ⁶³Ni by the semipermeable membrane to 90% at pH 8, which may be associated with the formation of mixed Fe(III)–Ni(II) complexes. In the presence of strong complexing agents, ⁶³Ni forms mainly negatively charged complexes with ethylenediaminetetraacetate ions. In boric acid solutions at pH 3–9, ⁶³Ni exists as positively charged species. At pH increased to ≥9.5, negatively charged nickel compounds are formed via both cation hydrolysis and complexation with negatively charged polynuclear boric acid species.</p>