Pacific water, sea ice meltwater, and river water are the primary sources of freshwater in the Arctic Ocean. We have determined their relative fractions on a transect across the Arctic Ocean Section 2005 Expedition onboard IB Oden, which took place from 21 August to 23 September 2005. The transect began north of Alaska, continued through the central Canada Basin to the Alpha Ridge and into the Makarov Basin, and ended in Amundsen Basin. Pacific freshwater and river water were the major sources of freshwater throughout the central Canada Basin and into Makarov Basin, with river water fractions sometimes considerably higher than Pacific water in the top ∼50 m. Pacific freshwater extended to depths of about 200 m. Pacific water found over the Alpha Ridge and in the Amundsen Basin is suggested to have been transported there in the Transpolar Drift. The inventories of Pacific freshwater and river water were roughly constant along the section through most of the Canada and Makarov basins. River water fractions were greater than those of Pacific freshwater in the Amundsen Basin. Sea ice meltwater fractions were negative (reflecting net ice formation) or near zero throughout most of the section. A comparison of freshwater inventories with those at stations occupied during expeditions in 1991, 1994, and 1996 indicated an increase in river water inventories in the Makarov and Amundsen basins on the Eurasian side of the Arctic Ocean.