In order to constrain the isotopic fingerprint of the Arctic hydrological cycle, a new generation of Picarro water isotope analyzers using CRDS (cavity ringdown spectroscopy) is used to monitor the isotopic composition of atmospheric water vapour at different sites. Since July 2015, within the ISOARC project water vapour stable isotopes (HDO and H218O; d excess) have been continuously measured: (1) on-board Polarstern (as oceanic moisture source) and (2) at Samoylov Island in the Lena Delta in northern Siberia (72°22’ N, 126°29’E) as sink in the water cycle. On Polarstern the water vapour has been sampled during ship cruise at 30 meters above sea surface. In addition to these water vapour measurements, ocean surface waters have been sampled on a daily basis and were later analysed in the laboratory for its water isotope composition. Data comprises two summer Arctic campaigns in 2015 and 2016 covering a large region of the Arctic Ocean, including the North Pole in September 2015. In the central and eastern Arctic Ocean, a large area of complete sea ice cover revealed a strong impact on the moisture above the ice cap under very cold conditions. On Samoylov Island, water vapour has been sampled from an inlet situated at 5 m height above the Siberian tundra typical for the Lena Delta. Isotope measurements were compared with meteorological and surface data collected throughout the year in the Lena delta, as well as to data from other sites such as Svalbard or Iceland run by partner institutes. We were able to measure the isotope composition to low humidity levels in Arctic winter. This water vapour network is a new approach into the understanding of the Arctic hydrological cycle at the regional scale. A first model-data comparison of our measurements with simulation results by the isotope-enabled atmospheric general circulation model ECHAM5-wiso have depicted relevant model biases in the Arctic realm, particularly close to the sea ice covered areas.