A mobile flotation facility, based on two 5 L minicells equipped with on-line monitoring of pH, dissolved oxygen and pulp potential, was used to study flotation of pyrite from zinc concentrate (i.e. reverse flotation) at the Trout Lake circuit of Hudson Bay Mining and Smelting Co. Ltd., Flin Flon, Manitoba. A combination of 2 levels of pH (pH 7 and 5.5 adjusted with sulphur dioxide) and temperature (60 degrees C and 80 degrees C controlled by steam) in conditioning and 3 levels of gas composition (air, 95% N-2, 100% N-2) in flotation were tested. For all conditions compared to the use of air, 95% or 100% N-2 gave higher pyrite recovery, Taking 3% sphalerite loss to the float product as a base line, nitrogen increased pyrite recovery by about 10% (absolute) even under the least severe conditions (pH 7 and 60 degrees C). In contrast, the flotation of chalcopyrite was retarded when using nitrogen.