Southeastern Europe is under the strong impact of ongoing climate changes, with noticeable effects on agriculture productivity, water supply security and status of water ecosystems. However, little is known in the region on the age, recharge patterns and overall availability of transboundary aquifers, some of which stretch the borders of Romanian, Republic of Moldova and Ukraine. The determination of age, timing and mechanisms of groundwater recharge are of paramount importance for the elaboration of scenarios of future water usage. In this endeavor, stable isotope techniques have proved to be one of the most reliable tools. The aim of this study is an analysis of stable isotope composition (?2H and ?18O) from the existing GNIP network and other sources in Romania and Republic of Moldova. Five GNIP stations from Republic of Moldova and five other stations from Romania and Republic of Moldova were analyzed for this task. All stations are situated east of the Carpathian Mountains in a region fed by precipitation derived mainly from the Atlantic Ocean, the Mediterranean Sea and the Black Sea, with additional continental sources in Europe and Western Asia. Local Meteoric Water Lines (LMWL) for each station was compared with Global Meteoric Water Line (GMWL) for the period 2009 – 2019 years. The monthly and annual fluctuations were studied in relationship with the main climate parameters as well as large-scale circulation patterns across Europe. Our results indicate that, while the Atlantic Ocean is the main source of moisture feeding the region, the Black and Caspian Seas contribute up to 25 % of moisture delivered as precipitation, a share that increases to above 30 % in winter months. In autumns, northeastward travelling Mediterranean cyclones provide moisture, especially in the southern part of the studied region, while in summer, locally evaporated moisture is the most important source of water. Atlantic Multidecadal Oscillation is the main largescale circulation pattern explaining the temporal and spatial distribution of ?2H and ?18O in precipitation; while in winter, a complex interplay between the Siberian High and the North Atlantic Oscillation influences the stable isotope patterns in precipitation. This study is a first step in the analysis of the interaction between precipitation, rivers, and the main transboundary aquifers between Romania and Republic of Moldova. This case study is the first assessment of the water sources in the frame of an ongoing IAEAsupported regional project (Romania, Bulgaria, Ukraine and Republic of Moldova).