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The order Thymo cretacei-Hyssopetalia cretacei Didukh 1989 combines chalk outcrop plant communities of the southwestern Central Russian Upland. Its specificity can be attributed to a rather peculiar and heterogeneous flora with a large number of endemic species. The question about its origin has caused a lively discussion, which has been going on since the late nineteenth century. Since 1989 works on the classification of these communities have frequently been carried out, but until today no unanimous decision could be reached. The purpose of our research was to conduct a critical analysis of the syntaxonomical structure of Cretaceous outcrop vegetation and to show its difference from the steppe vegetation of the class Festuco-Brometea Br.-Bl. et Tx. ex Soó 1947. The territory of our research covers the southwestern foothills of the Central Russian Upland and the Donetsk Range, located only within the steppe zone and characterised by Cenomanian chalk outcrops. In total 354 relevés were used for the analysis. The modified TWINSPAN classification was used for the analysis. Our research has shown that the order Thymo cretacei-Hyssopetalia cretacei includes twelve associations belonging to three alliances: Artemisio hololeucae-Hyssopion cretacei Romashchenko et al. 1996, Euphorbio cretophilae-Thymion cretacei Didukh 1989 and Centaureo carbonatae-Koelerion talievii Romashchenko et al. 1996.

Nine hundred and eight-nine relevés from calcareous petrophytic steppes in Ukraine and its adjacent territories were assessed with the help of expert systems to determine the syntaxonomic affiliation of the plant communities at class and order levels. At least 488 relevés belonging to the class Festuco-Brometea were analyzed using the TWINSPAN algorithm, and 8 distinctive clusters were obtained, recognized as alliances of the order Stipo pulcherrimae-Festucetalia pallentis. A new alliance, Bromopsido cappadocicae-Asphodelinion tauricae, was ascribed to the Crimean Mountains and the presence of two alliances, Diantho lumnitzeri-Seslerion albicantis and Genisto tetragonae-Seselion peucedanifoliae, was confirmed as new for this vegetation in Ukraine. Unlike in the Pannonian Basin, Bromo pannonici-Festucion csikhegyensis alliance communities mentioned in the literature do not occur in Ukraine. Centaureo carbonatae-Koelerion talievii has been provisionally transferred from Festucetalia valesiacae to the order Stipo pulcherrimae-Festucetalia pallentis. Furthermore, we distinguished alliances by their geographic locations and their climatic (thermoregime, cryoregime, light in communities) and edaphic (carbonate content, salinity, and acidity) features.

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