A large scale Europe-wide proficiency test (REM 2018 PT) on the determination of the massic activity of 222Rn in drinking water was organised by JRC-Geel. The 101 participating environmental radioactivity monitoring laboratories were either nominated by their corresponding national authorities or invited by JRC to participate. Spring water with an elevated 222Rn massic activity was used as the proficiency test material. The water was collected from a natural water source in Austria. Since the material for the PT was collected at a location more than 900 km away from the organiser’s laboratories, it was challenging to execute the packaging and transport in a timely manner with short storage and delivery time. We conducted a PT at a smaller scale (Jobbágy et al., 2019b) to test, optimise and streamline the complete process before running the Europe wide PT. The acquired experience helped us to perform in a robust manner the large scale Europe-wide PT with an increased number of participants. The process involved using state-of-the-art knowledge for the sampling and transport of water containing 222Rn (e.g. submersion sampling, temperature controlled transport, radon tight transport containers, etc.). A rapid homogeneity screening was performed immediately after sampling, showing a stable massic activity of 222Rn in the samples covering the whole sampling period. For the reference value determination, homogeneity and short term stability study, two independent measurement methods were used: gamma-ray spectrometry and liquid scintillation counting. The assigned reference value was established by taking the power-moderated mean of the gamma-ray spectrometry measurement results. The uncertainty of the reference values includes the uncertainty related to stability, between-bottle homogeneity, characterisation measurements and sampling. The performance of each participating laboratory was evaluated with respect to the reference value using relative deviations, z-score, zeta-score, En-number. Additionally PomPlots were made up to visualise the data. The results were evaluated grouped by the analytical methods used, to check for method dependency. It was found that the majority of the participants’ results, 113 out of 135 (84%), were within the acceptance range, which was set to ± 20% of the reference value. This suggests that each of the applied methods (emanometry, gamma-ray spectrometry and liquid scintillation counting) seems to be adequate for radon-in-water measurements. However, when the reported value with its uncertainty was evaluated (zeta score), fewer acceptable scores were found, 103 results out of 135 (76%). A key problem with 222Rn is that almost all errors and problems that can occur lead to an underestimation of the massic activity as the inert 222Rn gas can escape at any stage, from sampling until measurement