Abstract: The Oligocene sedimentary sequence of the Taatsiin Gol and Taatsiin Tsagaan Nuur area is of unique stratigraphic importance: here, the exposed sediments of the Hsanda Gol and the Loh Formations display multiple fossil horizons and interbedded basalt layers. In the frame of a Mongolian–Austrian project, 289 fossil taxa (11 Gastropoda, 2 Amphibia, 9 Reptilia and 267 Mammalia) were collected from 85 fossil horizons of 33 sections/fossil sites of the study area. The taxa were identified by an international team of specialists. This contribution presents comprehensive mammal lists of all localities. By integrating the new data on large and small mammals, the Mongolian informal biozones A, B, C, C1 were updated. 40Ar/39Ar-datings provide at least two groups of basalt ages, the Early Oligocene basalt I group around 31.5 Ma and the Late Oligocene basalt II group around 28 Ma. They serve as chronological tie points in the Oligocene mammalian stratigraphy. From the Early to the Late Oligocene the mammal associations underwent remarkable changes involving a significant decrease of species numbers in the Late Oligocene. This trend was most striking in creodont, carnivore and ruminant communities.