Key comparison (KC) CCQM-K18.2016 was performed to evaluate the degree of equivalence between pH measurement results on an unknown carbonate buffer reported by participating National Metrology Institutes and Designated Institutes (NMI/DIs). The participants used the highest metrological method existing at their institution. The nominal buffer pH was 10.0 and was determined only at 25 °C. Good agreement of the results is demonstrated by most participants. Main text To reach the main text of this paper, click on Final Report . Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/ . The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).