The effects of two large experimental crude oil spills conducted in the winter and summer 1976 in a permafrost-underlain black spruce forest of interior Alaska were assessed 15 years after the spills. Effects on permafrost, as determined from measurements of active layer thaw depths and of the total amount of ground subsidence, were far more pronounced on the winter spill due to a larger surface-oiled area. The winter spill also had a more drastic effect on the vegetation. Where the black, asphalt-like surface oil was present, black spruce mortality was 100% and there was very little live plant cover except for cotton grass tussocks. Changes in oil chemistry varied with depth; surface samples had signs of microbiological degradation, whereas some subsurface samples taken just above the permafrost had no evidence of degradation and still contained volatile fractions.