Abstract The problem of fine particle gas cleaning is especially actual at the present time. This is due to the need to capture the desired product and waste gases cleaning. In both cases, the equipment should provide maximum collection efficiency. Cleaning of gas from thermal power plant (TPP) fly ash is serious problem. Coal-fired thermal power plants provide 27% of the total world energy consumption. Conventional TPP produce more than 700,000 tonnes of fly ash per year. Annually fly ash production only in Russia, according to various estimates, is 27–35 million tons. Various apparatus are used to clean the gases from the fly ash. Required collection efficiency of purification units must to be 99.5–99.7% and higher. Wet scrubbers makes ash recycling more difficult. It requires ash separation from the slurry and drying. One solution of gas cleaning problem is a electrocyclone. It provides gas purification efficiency up to 99.9% at initial ash concentration equal 50 g/m 3 and higher. An electrocyclone allows to obtain a product in dry form. Re-entrainment is return of captured material to clean gas stream. Re-entrainment in the gas cleaning equipment is one of the negative effects. Re-entrainment reduces equipment efficiency at high gas velocities. The present study was carried out to determine the value of re-entrainment in the electrocyclone. The object of study was the electrocyclone of ‘pipe in pipe’ type. Aluminosilicate fly ash from thermal power plant (TPP) was the test material. Study was carried out in dry and wet modes. Re-entrainment was observed in dry operation mode. No re-entrainment was observed in wet operation mode. The value of re-entrainment was calculated. It decreases collection efficiency from 99.9% to 60%. Re-entrainment depends on aerosol velocity (range 14–27 m/s) and the aerosol concentration (range 2–30 g/m 3 ). It is shown, what re-entrainment can be eliminated by water irrigation of collecting electrodes.