The aim of this work is a theoretical analysis of the deposition of charged aerosol particles in a fibrous electret filter under nonsteady-state conditions. In this case loading of the filter's collectors with deposits causes two oppositeeffects: growth of the dendritic structures on the collectors’ surfaces results in an improvement of the deposition efficiency due to inertial impaction, sedimentation and Brownian motion, whilst it simultaneously leads to a decrease of the efficiency corresponding to the electrostatic attraction as a result of weakening of electric field around an electret fiber becoming more and more loaded with the deposits, having the opposite electric charge. At the same time, the second of the most important parameters of the fiber (in addition to its efficiency of the dust removal) -i.e. the pressure drop, increases continuously with the filter loading. Moreover, it should be remembered, that loading of the filter is -in a principle-non-uniform along it. Hence, the variation of the filter quality factor, QF, in time may be different for various operating conditions. The simplified model formulated, which takes into account the local changes of the gas flow pattern and electrostatic field intensity around a loaded collector, allows to study the course of the process, and it may be useful for optimization of gas-cleaning filters that make use of electrostatic interactions. The description of the mathematical model formulated is as complete as passible to make it easily adaptable for all specific situations. Some results of numerical simulations obtained are discussed.