Changes in the porous structure of fly ashes ZS-13 subjected to chemical treatment with solutions of NaOH, NaOH/NH4HCO3, EDTA, and HCl are examined. The initial material with 3 m(2)/g surface area and 0.01 cm(3)/g pore volume after modification revealed an increased surface area and pore volume. The greatest increase in the surface area equal to 105 m(2)/g was noted for the sample treated with HCl. while the greatest increase in pore volume equal to 0.18 cm(3)/g was obtained for the sample treated with EDTA. In the modified products, meso-and macropores prevail, although in the sample treated with HCl, the presence of a small contribution of micropores was detected. All the modifications, except that treated with HCl, were found to contain slit-shaped macropores. In the sample treated with HCI, the pores were in the shape of nonparallel plates. A study by scanning electron microscopy (SEM) allowed a detailed determination of the shape of the isles and agglomerates. Chemical treatment of the initial fly ashes resulted in a transformation of the ball-shaped agglomerations into different products characterised by specific size and shape of particles.