Effect of calcium and thiosulfate ions on flotation of a Ni-Cu ore was investigated, The ions improved the notability of sulfides at the normal process pH after grinding in steel mill. Calcium activated especially nickel and copper sulfides when the galvanic effect of mill iron was effective, The UV/Vis measurements showed that calcium ions increased xanthate adsorption oil the sulfides. The effect Of thiosulfate was different. It was concluded that thiosulfate reduced the effect, of hydrophilic compounds on sulfide particles, and in that way improved their flotation, (C) 2002 Elsevier Science Ltd. All rights reserved.