Provided herein is technology relating to calorimetry and particularly, but not exclusively, to apparatuses, methods, and systems for making high-resolution thermodynamic measurements of reactions between gas phase reactants and nanomaterials. For example, the technology can provide thermodynamic measurements with a high heat flow resolution and long term stability at a wide range of temperatures and reaction pressures. The technology is used, for example, to study the thermodynamics of surface reactions and phase transformations in nanomaterials.