Main text This pilot study evaluates the coherence of carbon isotope delta reference materials (RMs) used by laboratories worldwide. The goal was to assess how well laboratories can realize the VPDB scale using these RMs. In total, carbon isotope delta values of 36 RMs were measured by seven laboratories as well as the vanillin sample previously evaluated in CCQM-K167/P211. A hierarchical Bayesian measurement model was used to evaluate the coherence between these RMs, and to determine the carbon isotope delta value of each of these RMs. In general, this study shows that the 36 carbon isotope delta RMs are reliable. Further, the value assigned to the vanillin sample in this study is within 0.02 ‰ from the CCQM-K167 KCRV. 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 https://www.bipm.org/kcdb/. 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).