[en] The Analytical Laboratories for the Measurement of Environmental Radioactivity (ALMERA) network established by the IAEA in 1995 makes available to Member States a worldwide network of analytical laboratories capable of providing reliable and timely analysis of environmental samples in the event of an accidental or intentional release of radioactivity. The network is a technical collaboration of existing institutions. It provides an operational framework to link expertise and resources, in particular when a boundary-transgressing contamination is expected or when an event is of international significance. A primary requirement of the ALMERA members is participation in the IAEA interlaboratory comparisons which are specifically organized for ALMERA on a regular basis. These exercises are designed to monitor and demonstrate the performance and analytical capabilities of the network members, and to identify gaps and problem areas where further development is needed. Continued membership has benefits in training and educational opportunities, Enhanced mutual trust in results and methodology and objective evidence for accreditation purposes. The performance evaluation results of the interlaboratory comparisons performed in the frame of the ALMERA network are not anonymous for those laboratories nominated to participate as ALMERA members. The Po-210 poisoning event which occurred in November 2006 brought into focus a number of issues, including the capacity of laboratories to rapidly and accurately determine this radionuclide in environmental samples. A number of requests were received from ALMERA members to address this issue. Responding to these requests, the Chemistry Unit of the Physics, Chemistry and Instrumentation Laboratory in the IAEA's Seibersdorf Laboratory in Austria, conducted a proficiency test in the frame of the ALMERA network on the determination of Po-210 in water. The aim was to gather information on the current state of practice for Po-210 measurements at various levels in aqueous samples. This report describes the methodology employed and the results obtained in this proficiency test