Results of hydrological and geological investigations carried out during the last ten years in the deserts of the Sinai and the Negev indicate that unique climatological and hydrological conditions prevailed between ca. 100,000 and ca. 10,000 yr B.P. The uniqueness was expressed by the following characteristics: 1. (1) Higher precipitation and recharge rates caused the Nubian Sandstone aquifers to be filled up through the outcrops in central Sinai and to overflow through big springs emerging along the faults of the Syrian-African rift system. This water is of special isotopic composition, namely depleted in 18O and deuterium and with a lower excess of the latter. 2. (2) Dust-laden rainstorms prevailed, probably mainly during the autumn season, causing the deposition of thick layers of loess. The quantity of rain during the humid periods was double that of the present. 3. (3) The drainage system of many western and northern wadis of Sinai as well as the Negev could not drain out all the silt-loaded water during the rainy season, and shallow lakes and marshes extended in many wadis during this season. 4. (4) The floods and the springs flowing into the Dead Sea rift valley contributed to the Lisan Lake, which was 200 m above the level of the contemporary Dead Sea.