Temperature, salinity, nutrients, oxygen, and halocarbon data collected in the Arctic Ocean reveal a frontal structure previously unrecognized in the hydrography of the Canadian Basin. Samples were collected on a 1300-km section extending from the Beaufort Sea in the Canada Basin to the East Siberian Sea in the Makarov Basin. These data, collected in 1993 aboard the CCGS Henry Larsen, reveal a lateral boundary between water masses of Atlantic and Pacific origin. The term water mass assembly is introduced to describe the basic arrangement or vertical stacking of water masses found in the Arctic Ocean, recognizing that water mass components within each assembly may differ from basin to basin. Using historical data, two primary water mass assemblies are defined, each consisting of three layers: an upper layer, an Atlantic layer, and a deep layer. These two assemblies are marked by important differences. One assembly, here defined as the Western Arctic (WA) assembly, is characterized by an upper layer of relatively fresh, high-nutrient water of Pacific origin; below this, by an Atlantic layer with a core temperature generally below 0.5 ¿C; and finally, by a deep layer of higher salinities and colder temperatures (about -0.5 ¿C) than found in the overlying Atlantic layer. The second assembly, here defined as Eastern Arctic (EA) assembly, is characterized by the absence of Pacific water in the upper layer; below this, by an Atlantic layer core as warm as 2¿ to 3 ¿C; and by a colder (about -0.9 ¿C) deep layer. Because the presence or absence of Pacific origin water is a key characteristic distinguishing the two assemblies, we will refer to the water mass boundary between the two assemblies as the Atlantic/Pacific front. Earlier research indicated that water masses in the Arctic Ocean were separated by a front above the Lomonosov Ridge into the Canadian and Eurasian basins. Although all Larsen-93 stations from the Canada Basin (A1-D1) display classic WA assembly characteristics, the Makarov Basin station (E1) shows EA assembly characteristics in the upper and Atlantic layers and a WA assembly deep layer. This suggests a relocation in the position of the Atlantic/Pacific boundary away from the Lomonosov Ridge. Further, Larsen-93 data show the transition region between the Atlantic and deep layers is fresher in the Makarov Basin than corresponding water in either the Canada or Eurasian basins, implying a source of cold, low-salinity water, perhaps from the Laptev and East Siberian shelves. The front separating these two assemblies lies above the Mendeleyev Ridge and is marked by large lateral gradients in all measured properties. In particular, the penetration of anthropogenic halocarbons is 2 to 3 times deeper in the Makarov Basin than in the Canada Basin, implying enhanced rates of ventilation. Thissuggests that direct exchange between the Canadian and Eurasian basins has occurred recently near the perimeter and that physical and chemical properties, including contaminants, may have been transported by boundary currents more quickly from one basin to the other. ¿ American Geophysical Union 1996