An interlaboratory comparison was organised by the Institute for Reference Materials and Measurements (JRC-IRMM) among environmental radioactivity monitoring laboratories for the determination of gross alpha/beta activity concentration in drinking water. Four independent standard methods were used for the determination of the reference values of three different water samples. The performance of participating laboratories was evaluated with respect to the reference values using relative deviations. Sample preparation and measurement methods used by the participating laboratories are described in detail, in particular in the view of method-dependency of the results. Many of the participants’ results deviate by more than two orders of magnitude from the reference values regardless of the techniques used. This suggests that gross methods need revision. ; JRC.D.4-Standards for Nuclear Safety, Security and Safeguards