This work presents the results from the first investigation of Br and Cl isotopic partitioning in Southern Ocean sea ice. The sea ice samples were collected during the Australian-led Sea Ice Physics and Ecosystem eXperiment-2 (SIPEX-2) marine voyage in austral spring of 2012 (26 Sept - 10 Nov), within the area bounded by 115-125 degrees E and 62-66 degrees S off the East Antarctic coast. Multicollector inductively coupled plasma mass spectrometry was used to provide isotopic data for bromine and chlorine. Bromine concentrations ranged from 710 +/- 20 mu g L-1 to 31440 +/- 570 mu g L-1 and chlorine concentrations ranged from 218 +/- 12 mg L-1 to 7945 +/- 351 mg L-1. Bromine and chlorine concentrations were positively correlated with a slope of 0.0038, consistent with the seawater Br/Cl ratio of 0.0035. Bromine isotopic compositions (denoted delta Br-81 with respect to SMOB isotopic reference) varied from -0.64 to 0.10 parts per thousand with a mean of -0.16 parts per thousand and standard deviation of 0.04 parts per thousand. Chlorine isotopic compositions (denoted delta Cl-37 with respect to SMOC isotopic reference) varied from -1.10 parts per thousand to 0.43 parts per thousand with a mean of -0.26 parts per thousand and standard deviation of 0.10 parts per thousand. Despite their strong concentration correlation, there was a much less consistent relation between Br and Cl isotopic compositions and great variability within individual sea ice cores. The samples showing the largest degree of fractionation in their Br and Cl isotopic compositions were found deepest in the sea ice samples. Multiple parameters may influence isotopic fractionation processes in sea ice, including ice texture, age, salt content, and algal biomass content (represented by chlorophyll-a concentrations), although no single factor was found to consistently correlate with the bromine or chlorine isotopic signature. We consider the possibility of isotopic fractionation resulting from brine rejection and desalinization processes. (C) 2020 Elsevier Ltd. All rights reserved.