The requirements for gas purification systems are getting increasingly stricter. This is due to the development of new technologies, increasing environmental pollution, and greater emphasis placed on the protection of human health and the environment. Fibrous filters are among the most widely used tools for removing particles from fluids. This paper explains the very complex process of the non-steady state aerosol particle filtration in fibrous filters using such a technologically advanced solution as an X-ray computed tomography for this purpose. Since online monitoring of the process inside the filter structure or marking particles flowing into the filter in any way, which would facilitate distinguishing between fresh and replaced particles, might be difficult to deploy, a simulation of the process was chosen instead. The experiment was divided into four stages. At the first stage, the advantages of the melt-blown technique were adopted to produce two non-woven fabric types of a given structure - one made of micro-sized fibers, and the other made of submicro- and nano-sized fibers. At the subsequent stage, the provided separating materials were loaded with solid silica particles in identical conditions, and then a stream of clean air at various flow velocities was blown through them. This experiment was designed to examine the behavior of the deposits in the filter during filtration, observe whether they travel inside the filter structure, remain in place or entirely leave the filter. At the last stage of the process, a macroscopic analysis of the filter structure and silica particles deposited inside it was carried out using an X-ray microtomography system. Based on the acquired tomography images, profiles of changes in local porosity of the filters along their depth were made. It was found that deposited particles re-entrain from the fibers during filtration process but they do not leave the filter, only move to its deeper parts. (C) 2015 Elsevier B.V. All rights reserved.