The active layer of three distinct environments (undisturbed forest, moderately disturbed right-of-way, and severely disturbed trench) was examined during each of the 3 years following an experimental crude-oil spill in a black spruce forest in the Northwest Territories. The first year after the spill, the active layer in the oiled forest increased by >150%, in the oiled right-of-way by >80%, and in the oiled trench it did not change. By the third year, the active layers in all oiled environments were significantly deeper than their unoiled counterparts, and the active layer in the oiled trench was significantly deeper than that in the oiled forest. When compared with other studies for a crude-oil spill in a subarctic forested environment, this oil spill caused the greatest increase in thaw depths. This dramatic and persistent increase in thaw depth was likely a function of the high oil concentration, especially in areas where oil had pooled on the ground.