The technique of microelectrodes in a non-conducting wall was used to determine the flow regimes in a trickle-bed reactor and to characterize the wall wetting by the analysis of the Probability Density Functions (PDF). The identification of flow regime transitions (trickling/pulsing, trickling/dispersed and dispersed/pulsing) and the study of the wall wetting are carried out respectively by the analysis of the variations of the fluctuation rate of the local liquid/solid mass transfer coefficient and the wetting rate as functions of the gas and liquid flow rates.