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In the paper, the effect of the pulse electron radiation on the stability of the colloidal iron solutions is studied. The work is performed on the standard test solutions which correspond to the real natural and waste water regarding their chemical composition. The irradiation was performed using the pulse accelerator TEA-500 with the electron energy 500 keV and pulse duration 60 ns. The dose was calculated using the potassium nitrate recommended for measuring the dose of the pulse electron radiation. The standard test solution used for radiation contained iron, silicon and humic substances ions in the correlation 5.6:20:2.2 mg/l, respectively. The solutions were subjected to radiation within the range of 0.6-5.0 Mrad. The efficiency of radiation was estimated by the variation in iron chromaticity and concentration in the initial and irradiated solution. The decrease in iron concentration and solution chromaticity after the radiation indicates the violation of the stability of the colloidal solution and the Fe(OH)3 coagulation sludge. To explain the interaction of the radiation with the colloidal particles, the water radiolysis mechanism was considered, since the most of the ionizing radiation energy is consumed for the interaction with the water forming a large spectrum of radiolysis products. In the paper we offer the interaction mechanism of hydrated electron and the colloidal particles on the ground that the hydrated electron has a much longer life compared to H and OH radicals and is formed with a large radiation-chemical output which is equal to 3.3÷3.5 ions per 100 eV. The lifetime of the hydrated electron is 2.3*10-4 s. Besides, the size of the hydrated electron is 0.38 nm that is much less than the size of micelle and because of these factors it interacts with the micelle nucleus. According to the literature data, the mechanism of the electron transfer to the micelle nucleus is accompanied with the violation of the double elec- rical layer, the destruction of the micelle forming the Fe(OH)3 sludge that was observed during the electron beam irradiation of the standard test colloidal solutions. In the paper we present the results on the effect of the ion nitrate on the kinetics of the variation in iron ion color and concentration in the standard test solution.