In the Strathcona Mill, the rejection of pyrrhotite (F(1-x)S, 0 < x <= 0.125) is in high demand to meet the stringent environmental standards for SO2 emission in downstream smelting operations. For this purpose, two plant surveys were conducted to study polymorphic pyrrhotite flotation and depression mechanisms in the Strathcona Mill via investigating polymorphic pyrrhotite distributions, pulp chemistry, and industrial settings (with combined Cu-Ni feed and split Cu-Ni feed). The plant survey data revealed that hexagonal pyrrhotite in the Ni concentrate demonstrated higher floatability than that of the monoclinic pyrrhotite. Surface analysis by ToF-SIMS of the primary nickel rougher circuit showed that lower copper activation effects were likely responsible for the decreased flotation recovery of pyrrhotite in the 2017 plant survey as compared with combined Cu-Ni feed in 2016 survey. The different oxidation rates of monoclinic and hexagonal pyrrhotite were also investigated. CV studies corroborated the higher floatability of hexagonal pyrrhotite with its lower oxidation rate than monoclinic pyrrhotite. XPS studies demonstrated the progressive oxidation of pyrrhotite from concentrate to tail, substantiating that pyrrhotite oxidation is essential for pyrrhotite depression. This study suggested that careful control of Cu ions in the Ni stream can depress polymorphic pyrrhotite in the flotation. The oxidation study of pyrrhotite demonstrated that proper ways to relatively increase pyrrhotite oxidation rate (particularly for hexagonal pyrrhotite) would help to depress pyrrhotite.