Laboratory rougher flotation tests were conducted with two samples of Cu-Mo ores in fresh water and in sea water as a function of pH. In both cases Cu recoveries were slightly lower in sea water than in fresh water for a wide range of pH (pH 7-12). Flotation of molybdenite was however strongly depressed in sea water at pH higher than 9.5. Frothers were characterized by measuring froth thickness in a modified laboratory flotation cell as a function of pH, salinity, frother type, and solids content (%). It was found that for all tested frothers, foamability in two-phase systems was better in sea water than in fresh water. However, froth layer thickness measurement showed that frothability also depends on solids content and increases with increasing pulp density. At high solids content (35%) the frothability depended strongly on pH. At pH of 9 it was similar for fresh water and sea water, however, once the pH is raised further frothability increases sharply when the tests are carried out in fresh water, but this was not observed in sea water. The froth in sea water is drier than in fresh water, and this probably also depends on solids content. (C) 2013 Elsevier Ltd. All rights reserved.