Relevance of the discussed issue. Typical siliceous section of Trans-Urals has a threefold structure as a rule: silicified opoka of the serov formation occur at the bottom of the section, while diatomite and diatomaceous clays of irbit formation are deposited above. The main industrial deposits of diatomite in the region (Irbit, Kamyshlov, Potaninskoe deposits etc.) are confined to the lower Eocene Irbit formation, which is traditionally observed as the main productive horizon for this type of mineral raw materials. At the same time, development of the diatomite rock mass in Trans-Urals started in the terminal stage of the Paleocene, which is identified by the presence of diatom complex Trinacria ventriculosa-Sheshukovia mirabilis. Up to the present date the particular features of the lithology of serov formation Paleocene diatomites have not been practically studied, there is no information on clearly identified outcrops of serov diatomite within Trans-Urals, the industrial potential of these deposits has not been determined. The main aim is a comparative study of the lithology, chemical and mineralogical composition of the Eocene Irbit formation diatomite (Kamyshlov quarry) and upper Paleocene Serov formation diatomite (section Brusyana). The methods used in the research: field studies, X-ray diffraction, X-ray fluorescence analysis, infrared spectroscopy, and inductively coupled plasma mass spectrometry, scanning electron microscopy, lithological and petrographic analysis. The results. For the first time the paper introduces the results of the complex studies of chemical and mineralogical composition, features of microstructure and lithology of the Paleocene diatomite from Trans-Urals on the example of rocks occurrence in the section Brusyana (Sverdlovsk region). It was found out that Paleocene diatomite form section Brusyana predominate over the Eocene diatomite and diatomaceous clays of most deposits in Trans-Urals in terms of SiO2 content, the degree of diatom fossils preservation and microporosity and, accordingly, may be considered as a promising object for further exploration. Paleocene diatomite of serov formation is characterized by a higher content of glauconite and terrigenous material (up to 10 %), which, apparently, identifies activation of erosion within the sedimentary basin during the early stages of diatomite rock mass formation in Trans-Urals.