In this paper, NMR imaging was applied to study the distribution of the liquid phase in a fixed catalyst bed of an operating multiphase reactor with a cocurrent gas-liquid flow under conditions when the heterogeneous catalytic hydrogenation of 1-octene was taking place in the reactor. The 2D maps of the spatial distribution of the external and internal liquid holdups in the various regimes of the catalyst bed operation have been obtained for the first time in the course of the reaction. It was shown by a direct in situ method that an exothermic reaction taking place in a trickle bed reactor dramatically affected the distribution of the liquid phase in the catalyst bed. In particular, in the presence of the reaction, the catalyst bed was characterized by a non-uniform distribution of the liquid phase, as revealed by evaluating the external and internal liquid holdups. In the absence of the reaction, the bed was largely filled with the liquid phase. (C) 2011 Elsevier B.V. All rights reserved.