The most important requirement, which must be met by a material for an isolation barrier is its permeability. Permeability is characterized by filtration coefficient, which value must be not higher than 1ź10-9 m/s. Active porosity of a material decides on the value of filtration coefficient that means pores of diameter larger than 0,2 ?m. Open porosity (no) of stabilized mixtures of fly ash with desulphurization by-products from power plant "X" and water is equal in average to 20,41%, and its active porosity (na) 3,04% (see Table 2 and Fig. 2). Open porosity n0 regards on sub-capillary pores in a range of diameter 0,01-0,1 ?m. Filtration coefficient measured for discussed material equals, in relation to a method used, 1,55 10-10 m/s for Kamienski permeameter and 1,81 10-9 m/s for the flow pump method (see Table 3 and Figures 3 and 4). Obtained values of porosymetric properties and low filtration coefficient of mixture of fly ash with desulphurization by-products from "X" power plant prove positively on ability to use of this type of fly in different technologies related to isolation of structures, i.e. isolation barriers for underground waste disposal sites, sealing plugs and dams, shortcretes, etc.