Sea ice brines were collected from a single floe composed of different ice types in the western Weddell Sea in December 2004. The chemical composition of the brines (temperature: -3.4°C to -2.1°C; salinity: 40-63) was examined on seven occasions over 25 days with measurements of dissolved oxygen, dissolved inorganic macronutrients (nitrate plus nitrite, ammonium, phosphorus [DIP], and silicic acid), pH, total alkalinity (AT), dissolved organic carbon (DOC) and nitrogen (DON), total dissolved inorganic carbon (CT), and the stable isotopic composition of CT (δ13CT). The in situ pH ranged from 8.41-8.82 on the seawater scale, dissolved oxygen from 212-604 μmol kg-1, nitrate from 0.1-3.1 μm kg-1, ammonium 0.1-2.4 μmol kg-1, DIP 0.4-2.0 μmol kg-1, silicic acid 4-80 μmol kg-1, AT 2,690-4,620 μeq kg-1, DOC 115-359 μmol kg -1, DON 8-26 μmol kg-1, CT 2,090-3,550 μmol kg-1, and δ13CT +2.9‰-+6.4‰. Compared with the chemical composition of surface oceanic water (salinity of 34), the brines had elevated pH, reduced concentrations of dissolved inorganic macronutrients (including carbon), especially dissolved inorganic nitrogen, and were mostly supersaturated with dissolved oxygen with respect to equilibrium with air, whereas the CT was considerably enriched in 13C. The chemical composition of the brines was consistent with internal biological productivity, but there was a lack of a distinctive and uniform relationship among the major dissolved inorganic nutrients typically used for describing biological activity. This was interpreted as the result of varying stoichiometry of biological activity within a very small spatial scale. Modification by abiotic processes was a potential contributing factor, such as degassing acting on the dissolved oxygen concentration. Carbonate mineral formation, acting on AT and C T, was not evident in brines from first-year ice but was apparent in brine from second-year ice. © 2007, by the American Society of Limnology and Oceanography, Inc.