FORMULATION OF A PROBLEMTrack-etched membranes (TMs) are polymer films with a system of ultrafiltration or microfiltration pores formed by the irradiation of initial (nonporous) films with the flux of high-energy particles and subsequent chemical treatment [1][2][3][4][5]. The TMs are usually produced on the base of the films of polyesters (polycarbonate and poly(ethylene terephthalate)) with the thickness of 5-25 µ m. The pores are formed due to the etching of tracks (the regions of the damaged initial structure of a polymer formed after the charged highenergy particles passed through polymer film) by the alkaline solutions. Accelerated heavy multi-charged ions or nuclear fission fragments are used more often as such particles.The sizes of TM pores usually ranged from 50 nm to 5 µ m; the number of pores per surface unit (pore surface density) varies from 10 6 to 10 9 cm -2 that corresponds to the values of bulk (or surface) porosity 1 from 5 to 10% (for commercial TMs, this value is usually larger, 10-20%) [5]. The TMs differ from ultra-and microfiltration membranes produced from polymer solutions or by the molding of finely dispersed powders mainly by a relatively small scatter of pore sizes (the deviation from the mean value does not usually exceed ± 5% ) [6]. As a rule, the pore shape in polymer TMs is close to cylindrical.The main requirement for TMs is a low scatter of shape and surface density of the pores at the stability of these parameters under the operating conditions, their prolonged storage, possible subsequent treatment (e.g., 1 The porosity was determined as the product of pore surface density by the cross-sectional area of a single pore.Abstract -Atomic force microscopy (AFM) is employed to obtain information on the main stages of the preparation of ultrafiltration track-etched membranes based on poly(ethylene terephthalate) (PET). The surface structure of initial commercial samples of PET films and the same films irradiated with various fluxes of accelerated heavy ions and subsequently treated with alkaline that results in the formation of pores owing to track etching is studied. It is shown that the order of the aforementioned stages of the formation of track-etched membranes with various porosities (from 0.02 to 6%) at a typical pore size of about 50 nm leads only to slight changes in surface structural parameters and does not fundamentally affect the polymer structure formed by spherulites with sizes that are comparable with the pore sizes. In this case, nearly the same content of the crystalline phase in the initial film and track-etched membrane are identified by large-angle X-ray scattering. The picture of X-ray scattering by track-etched membranes at small angles fully corresponds to the scattering on cylindrical pores with a diameter of about 50 nm. The analysis of the set of roughness profiles of the surfaces of initial films and track-etched membranes obtained by the AFM technique makes it possible to determine and introduce-in addition to standard parameters of the surface, the mean v...