The influence of grinding media, process water ions (calcium and thiosulfate) and pulp oxidation on flotability of sulfides nickel ores was investigated. Bench scale flotation experiments were carried out with Enonkoski ore using ethyl xanthate as collector. The results showed that the galvanic effect of mill iron controls the separation selectivity of pentlandite from pyrrhotite. Calcium and thiosulfate ions activated the flotation of sulfides with xanthate after using a steel mill for grinding. The ions improved also the selectivity of nickel separation. After grinding in ceramic mill, the ions depressed sulfide flotation but increased the selectivity of nickel flotation in the presence of a collector. Pulp oxidation improved both the flotability of sulfides and the selectivity of nickel flotation with both grinding methods. (C) 2002 Elsevier Science B.V. All rights reserved.