text: Clumped isotopes present a distinct set of challenges for isotopic reference materials. By convention the clumped isotope composition of CO liberated from CaCO (Δ) and CH (Δ/ΔCH3DCH2D2CO for carbon dioxide and CH for methane. Standard isotopic reference materials are still required in clumped isotope analysis for measuring the bulk δ values of a sample, which are used to calculate the expected stochastic distribution and subsequently to infer clumped isotope temperatures. However, the clumped isotope composition of most samples are altered in the mass spectrometer during ionization, meaning that post-measurement data correction requires having measured materials of known clumped isotope composition. At a given temperature equilibrium thermodynamics predicts the distribution of isotopes among possible isotopologues, and these predictions can be calibrated experimentally. To generate a standard of known clumped isotope composition, current practice is to internally equilibrate analyte gas at high temperatures in the presence of a metal catalyst and assume that it has the Δ value predicted by equilibrium calculations. Developing isotopic reference materials specifically for clumped isotope analysis remains an ongoing goal of this rapidly developing field and was a major discussion topic during the 6th [http://www.ipgp.fr/en/iciw International Clumped Isotopes Workshop] in 2017. It is possible that researchers in the future will measure clumped isotope ratios against internationally distributed reference materials, similar to the current method of measuring the bulk isotope composition of unknown samples.