The uncertainty of absorption measurements in reverberation rooms is considered to be unacceptably large for certain applications. A long-discussed possibility to reduce the uncertainty is to measure the absorption relative to the known or defined absorption of a reference absorber. To check this approach, measured absorption coefficients from a large data base with round robin results are explored. This check is possible because several different specimen have been measured in the same set of reverberation rooms in some of the round robins. Furthermore, a special reference absorber has been proposed recently. This material was used for a comparison measurement at PTB. About 10 different laboratories came to PTB's reverberation room and measured the sound absorption of the proposed reference absorber using their own measurement equipment and data analysis. Additional investigations involved measurements of the reference absorber and some other specimens in two different reverberation rooms at PTB. Measurement and analysis results are presented and discussed in view of the current proposals for the revision of ISO 354.