[en] In order to achieve international acceptance of results, laboratories must be in a position to demonstrate that their results are reliable and traceable to SI units of measurement. Laboratories can provide objective evidence of their performance through participation in external quality assessment schemes sometimes referred to as proficiency tests (PT). To achieve this objective for environmental radioactivity laboratories in Spain, CSN, in collaboration with CIEMAT, has organised periodical intercomparison exercises to provide these laboratories with the means to assess the quality of their results. In order to implement the ISO/IUPAC/AOAC Harmonised Protocol for the Proficiency Testing of Analytical Laboratories, CIEMAT has been collaborating with the IAEA-AQCS during the last two exercises and has used IAEA reference materials as test samples. The first PT was organised using the IAEA-SED-1 which is a lake sediment containing K⁴⁰, Ra²²⁶, Ac²²⁸, Cs¹³⁷, Sr⁹⁰, and Pu-(²³⁹+²⁴⁰), while the second PT used IAEA-Soil- 6 for the same set of radionuclides. This paper presents the results reported for the second test as well as a comparison of the participants' performance in both tests. The application of this Harmonised Protocol has provided an objective assessment of the performance of the laboratories, enabling them to evaluate and make appropriate modifications to their procedures