Due to decreasing ore grades and increasingly complex ore bodies, the heap leaching of sulfide ores is of increasing interest. In this research, the heap leaching phenomena of a pyrrhotite-rich pentlandite ore was investigated for the first 150 days of operation in order to understand the phenomena taking place in the early phase of leaching in an industrial scale heap. The ore investigated originates from Terrafame (Finland) and contains approximately 20 wt% metal sulfides, the majority of which are pyrrhotite and pyrite. The nickel and zinc minerals of interest, pentlandite and sphalerite, are minor constituents of the ore. The oxidative leaching behavior of sulfidic minerals was shown to follow for the most part the order of nobility of sulfides. In the heap leaching investigated, pyrrhotite especially was shown to be far more reactive than pentlandite and sphalerite and thus the recovery of valuable metals can occur only after pyrrhotite dissolution. It was found that the nickel, zinc, and copper present in the irrigation solution can partially precipitate into the heap where more reductive metal sulfides, such as pyrrhotite and alabandite, are present, a metathesis phenomenon postulated elsewhere for copper and now confirmed in industrial scale operation. Pyrrhotite was also shown to create the majority of the reagent (acid and oxygen) demand and the majority of the reagent demand variation during the early days of heap leaching. In addition, the height of the heap was shown have an important role in determining the leaching time, due to physical, chemical, and practical limitations on acid feed. The acid demand for pyrrhotite leaching in low-oxygen consuming reactions was found to be higher (approx. 62 kg/tonne of ore) than the possibility to feed acid on a tall heap (23 kg/tonne of ore at maximum). This was shown to result in heavy precipitation of iron compounds, such as goethite. The results give more clarity for the initial stages of leaching at Terrafame and thereby enable clearer strategies to be formulated for solution circulation management according to heap age.