In August and September 1991, thickness, structure, and properties of level multiyear ice were studied at 66 locations in the Eurasian sector of the Arctic Ocean. The mean ice thickness was 2.86 m, with 0.31 m of freeboard (including 0.08 m deteriorated ice of mean density 370 kg m−3). On the basis of the study of ice cores, 61% of the ice cover consisted of undeformed columnar ice, the remaining 39% consisted of a mixture of ice types including frazil (18%) and deformed ice (9%). Through microstructural studies, six main classes of pores could be identified. The mean density of the ice cover increased from 720 kg m−3 at the top to >880 kg m−3 below 0.4 m depth. Sea ice salinities (mean value 2.1‰) correlate with ice thickness. On average, salinity profiles exhibit a linear increase from values close to 0‰ at the top to 2‰ at 1 m depth, with less steep salinity gradients below. Sampling from different depths within the ice cover indicates that the brine in summer sea ice is strongly stratified. The influence of meltwater percolation is evident, with salinities around 5‰ and pH values <8 at the top and >15‰ and >8 at greater depths. Brine volumes (ranging from 80 to 150‰) are controlled by the thermodynamic equilibrium between the solid and liquid phases. Gas volumes decrease from >200‰ at the top to <50‰ below 1 m depth. Pore microstructure is highly variable even on small scales. Salinity and other ice properties do not vary to a large degree between different regions. The evolution of level multiyear sea ice is discussed with particular reference to “hidden” occurrence of deformed ice and the importance of ablation processes.