Here, we present the results of the clumped isotope measurements on 20 Early Cretaceous (Valanginian) belemnites. Further, we provide the results of the equilibrated gases and the carbonate reference material that were measured along with the samples. Carbonate digestion at 90 °C and subsequent CO2 purification was performed on an automated line connected to a ThermoFisher MAT 253 isotope ratio mass spectrometer. The data is projected to the Carbon Dioxide Equilibrium Scale (CDES) and presented without an acid fractionation factor applied. Further details on the sample material, sample preparation method, and mass spectrometry procedures can be found in the associated files and the linked article: Price et al. (2020).