The S5 is the most prominent paleosol unit in the Chinese Loess Plateau, and is distinguished by its great thickness, dark color, and well-developed clay coatings. Previous studies led to the proposal that the S5 developed under much warmer and wetter conditions than at present, implying a “climatic optimum.” However, the S5 is a paleosol complex corresponding in time to a period of weak glacial–interglacial oscillations, as recorded by marine oxygen-isotope stages 15 through 13. To understand the character of this climatic interval, two profiles of the paleosol S5 in the Guanzhong basin were studied, with most of the emphasis being placed on the uppermost and best-developed pedon, S5-I. Grain-size, mineralogical, and chemical data indicate that the major pedogenic processes experienced by the S5-I included complete decalcification and mechanical translocation of fine particles. Chemical alteration of silicate minerals has been weak. Stable carbon-isotope analyses of organic matter showed that C4 plant remains are abundant throughout the profiles. These results suggest that grasslands dominated the paleovegetation during S5-I development. The prominent development of the S5 paleosol complex may reflect a prolonged period of pedogenesis. In contrast to previous reports, paleoclimatic conditions perhaps even drier than at present are implied by the data for the S5 profiles. The relative strength of the glacial–interglacial oscillation in this region (related to the East Asian monsoon) was similar to that indicated for global climate during S5 time.