Hugh O’Neill is innovative and excellent in combining the thermodynamics of minerals with advances in experimental petrology. He has advanced fundamental research in the systematics of spinel minerals, in the importance of redox equilibria in planetary processes, and in the composition of the Earth, Moon and planetary interiors. His quantitative measurements of thermodynamic properties of minerals are incorporated into thermodynamic databases, underpinning multidisciplinary research and used in applied mineralogy, modelling of ore deposits, petrology and global tectonics. Most recently, he has fostered and led the applications of synchrotron-based techniques to silicate melts, including redox equilibria and the solubility of sulphur and important trace elements.