This paper describes the design principles at the basis of the Round Robin Test carried out among the laboratories of the Sound Absorption working group of the Italian Standardization Body (UNI). In recent years, the attention to the determination of the measurement uncertainties in building and room acoustics fields raised up. Moreover, sound absorption has always been difficult to be determined, in particular for very highly absorptive materials, e.g. due to the edge effect, and the results coming from different laboratories cannot always be compared, due to differences in the actual sound field found in nominally equivalent facilities. During this RRT, carefully selected samples have been measured by the different laboratories participating to the RRT in their reverberation rooms. The procedure and the main results will be shown.