This work reports the first application of nuclear magnetic resonance (NMR) imaging to study the behaviour of a gas-liquid-solid model catalytic reactor operating at elevated temperatures. Two techniques from the magnetic resonance imaging (MRI) toolkit, namely functional MRI and NMR spectroscopic imaging, have been employed to perform a dynamic in situ study of alpha-methylstyrene catalytic hydrogenation on a single catalyst pellet or in a granular bed. Owing to reaction exothermicity, a reciprocating motion of the liquid front within the pellet accompanied by pellet temperature oscillations has been observed even when the external conditions were kept unchanged. Spatially resolved information on the reactant to product conversion within the catalyst bed has been obtained for a steady-state regime. The results represent a first direct MRI visualisation of the mutual influence of mass and heat transport processes and a chemical reaction in an operating reactor and demonstrate that NMR imaging and spectroscopy techniques can be used successfully to study the development of critical phenomena in operating multiphase reactors. We suggest that further development of the applications of the MRI toolkit in chemical and process engineering and catalysis hold significant promise for the development of safe and efficient technologies based on a detailed understanding of the interplay of various processes in an operating gas-liquid-solid reactor. (C) 2004 Elsevier B.V. All rights reserved.