The three-dimensional structure of a fibrous filter determines its behaviour during filtration, such as the obtained pressure drop, the fractional efficiency and the filter lifetime. Therefore, suitable parameters describing the inner structure (the local packing density) are key inputs for modelling filter performance accurately. The synchrotron X-ray holotomography, carried out at the European Synchrotron Radiation Facility, was performed in this study to provide information on the three-dimensional structure of a fibrous filter. Moreover, this non-invasive technique was used to determine the liquid distribution in a filter clogged with a liquid aerosol of di-ethyl-hexyl-sebacate. These samples were imaged at a voxel size of 0.7 mu m, which allows the quantification of the different phases within the filter (liquid, fibres and void space). The results complete the two-dimensional studies obtained by scanning electron microscopic and highlight the formation of liquid menisci not only on the filter surface but also in the filter depth. Image analyses of synchrotron X-ray holotomography radiographs also highlight a heterogeneous liquid distribution along the thickness of the clogged filter. (c) 2010 Elsevier Ltd. All rights reserved.