A large scale Europe-wide proficiency test (sometimes referred to as REM2019 PT) on the determination of the gross alpha/beta activity concentration in drinking water was organised by JRC-Geel. The 154 participating environmental radioactivity monitoring laboratories were either nominated by their corresponding national authorities or invited by JRC to participate. One spiked water sample (JRC-GAB2) and a commercially available natural mineral water (JRC-GAB1) were selected as reference materials for this proficiency test after initial testing using nuclide-specific analyses and gross alpha/beta measurements. The JRC-GAB1 reference material (mineral water) had intermediate mineral content and gross activity above the parametric values defined in the E-DWD (Euratom Drinking Water Directive). The original mineral water was collected from a natural water source in France. Reference values were established in collaboration between the JRC-Geel and the Belgian Nuclear Research Centre (SCK CEN). The homogeneity, short-and long term stability of the batch of distributed PT reference materials were checked, their contribution to the uncertainty of the reference value was assessed.