<p><strong>Abstract.</strong> The Lomonosov Ridge represents a major topographical feature in the Arctic Ocean which has a large effect on the water circulation and the distribution of water properties. This study presents detailed bathymetric survey data along with hydrographic data at two deep passages across the ridge: a southern passage (80–81<span class="inline-formula"><sup>∘</sup></span><span class="thinspace"></span>N), where the ridge crest meets the Siberian continental slope, and a northern passage around 84.5<span class="inline-formula"><sup>∘</sup></span><span class="thinspace"></span>N. The southern channel is characterized by smooth and flat bathymetry around 1600–1700<span class="thinspace"></span>m with a sill depth slightly shallower than 1700<span class="thinspace"></span>m. A hydrographic section across the channel reveals an eastward flow with Amundsen Basin properties in the southern part and a westward flow of Makarov Basin properties in the northern part. The northern passage includes an approximately 72<span class="thinspace"></span>km long and 33<span class="thinspace"></span>km wide trough which forms an intra-basin in the Lomonosov Ridge morphology (the Oden Trough). The eastern side of the Oden Trough is enclosed by a narrow and steep ridge rising 500–600<span class="thinspace"></span>m above a generally 1600<span class="thinspace"></span>m deep trough bottom. The deepest passage (the sill) is 1470<span class="thinspace"></span>m deep and located on this ridge. Hydrographic data show irregular temperature and salinity profiles indicating that water exchange occurs as midwater intrusions bringing water properties from each side of the ridge in well-defined but irregular layers. There is also morphological evidence that some rather energetic flows may occur in the vicinity of the sill. A well expressed deepening near the sill may be the result of seabed erosion by bottom currents.</p>