<p>In this study, the first fully-continuous monitoring of water vapour isotopic composition at Neumayer Station III, Antarctica, during the two-year period from February 2017 to January 2019 is presented. Seasonal and synoptic-scale variations of both stable water isotopes <i>H<sub>2</sub><sup>18</sup>O</i> and <i>HDO</i> are reported, and their link to variations of key meteorological variables are analysed. Changes in local temperature and humidity are the main drivers for the variability of δ<i>O</i><sup>18</sup> and δ<i>D</i> in vapour at Neumayer Station III, both on seasonal and shorter time scales. In contrast to the measured δ<i>O</i><sup>18</sup> and δ<i>D</i> variations, no seasonal cycle in the Deuterium excess signal d–excess in vapour is detected. However, a rather high uncertainty of measured d–excess values especially in austral winter limits the confidence of this finding. Overall, the d–excess signal shows a stronger inverse correlation with humidity than with temperature, and this inverse correlation between d–excess and humidity is stronger for the cloudy-sky conditions than for clear-sky conditions during summertime. Back trajectory simulations performed with the FLEXPART model show that seasonal and synoptic variations of δ<i>O</i><sup>18</sup> and δ<i>D</i> in vapour coincide with changes in the main sources of water vapour transported to Neumayer Station. In general, moisture transport pathways from the east lead to higher temperatures and more enriched δ<i>O</i><sup>18</sup> values in vapour, while weather situations with southerly winds lead to lower temperatures and more depleted δ<i>O</i><sup>18</sup> values. However, for several occasions, δ<i>O</i><sup>18</sup> variations linked to wind direction changes were observed, which were not accompanied by a corresponding temperature change. Comparing isotopic compositions of water vapour at Neumayer Station III and snow samples taken in the vicinity of the station reveals almost identical slopes, both for the δ<i>O</i><sup>18</sup>–δ<i>D</i> relation and for the temperature–δ<i>O</i><sup>18</sup> relation.</p>