Flotation tests using different size fractions of molybdenite with 50 ppm of Diesel oil and 25 ppm of MIBC, carried out in 0.01 M NaCl solutions and seawater, showed much lower floatability of the -10 mu m size fraction. The flotation of this finest size fraction was very much improved with the use of polyethylene oxide (PEO). The tests revealed that the improvement resulted from aggregation of fine molybdenite particles by Diesel oil-PEO emulsion. The flotation in seawater was good only in the pH range from 7 to 9 and it was strongly depressed around pH 10-11. While addition of PEO could improve flotation in the good flotation range over pH 7-9, it was not efficient in the depression range caused by precipitating magnesium hydroxy-complexes/magnesium hydroxide.