Clay mineral data relating to Quaternary sediments of mountainous and hilly regions of Hungary were collected from published papers and unpublished laboratory reports of the Hungarian Institute of Geology. Predominantly XRD and thermal data were considered. Mineralogy was compared with transport and environmental conditions of deposition of the pelitic material. There is an older, kaolinite-rich and a younger, smectite+illite-rich red clay type covering the karstic surfaces of Aggtelek Mts, Szendrõ Mts (N Hungary) and Villány Mts (S Transdanubia), indicating warm/humid and warm/arid climatic conditions, respectively. Clear relationships, however, may be confused by multiple redeposition of older weathering crusts. Volcanic ranges are covered by smectite (beidellite)-rich red clay (Mátra Mts and foothill areas) and loam (=nyirok, Tokaj Mts). In hilly areas of SE Transdanubia the Pliocene and Lower Pleistocene are represented by the Tengelic Red Clay Fm. which has similar compositional types as on the mountain surfaces. Traces of basalt tuffite (derived from the Bár volcano) are enriched in smectite and vermiculite. A special variety is the weathering crust of the Mórágy granite displaying transitional alteration products of biotite and chlorite in a Mg-rich environment. In the upper part of the Pleistocene the most typical fine-grained formation is loess, containing fresh, silt-size minerals, mainly illite and chlorite, detrital calcite and dolomite. Minerals in fossil soil layers in the loess indicate the climatic conditions of ancient weathering (neoformation of smectite, vermiculite and lesser kaolinite, leaching of carbonate).