Wurster fluidised beds are used in the pharmaceutical industry for pellet coating. Due to Wurster tube in the chamber, the flow hydrodynamic characteristics are complex and difficult to measure. This paper presents a 12-4-8 electrode electrical capacitance tomography (ECT) sensor used for investigation of complex gas-solids flow characteristics in Wurster fluidised bed coating processes by visualising solids concentration and distribution both inside and outside of Wurster tube synchronously. It was revealed that there exist seven flow regimes, i.e. bubbling-core, intermittent-core, plug-core, plug-bubbling, bubbling-dispersed, intermittent-dispersed and defluidisation, in a coating process. Further analysis of capacitance and pressure drop signals using a continuous wavelet transform (CWT) method indicates that the characteristic frequencies for bubbling, intermittent and plug flow in the annular region are 2, 0-1 and 0 Hz, respectively. The influence of different operating parameters and flow regime on the coating quality, in particular agglomeration, is discussed. Experimental results indicate that ECT is an efficient process analytic technology (PAT) tool to monitor the pellets coating process in a Wurster fluidised bed. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.