© 2015 University of Chicago Press. All rights reserved. The East Asian monsoon forms a significant element of the global climatic system. We here present detailed sedimentologicinvestigations of the Oligocene to early Miocene succession in the Sikouzi section, northern China, todetermine monsoon evolution during this time. The particle-size characteristics and grain surface structures, incombination with the large-scale cross-bedding arrangements, graded bedding laminations, and small-scale deformationstructures, type the Oligocene red beds in association I as eolian, attributable to large dune forms. Rare earthelement results imply that the Oligocene sediments are possibly sourced from a local provenance in the West QinlingMountains, excluding a more regional wind system during this time. Packages of thick-bedded dark-red mudstone/silty mudstone as well as subordinate medium-bedded pale-gray fine- to medium-grained sandstone and thin-beddedgypsum, with the massive mudstone/silty mudstone taking dominance, point to a lacustrine deposition with deeperlakephases taking dominance for the early Miocene association II. This significant transition in depositional environments,while possibly linked to local tectonic and/or global eustatic events, ismore likely to reflect a shift of regionalpaleoclimates from arid to more humid regimes. We attribute the shift to the impact of an active East Asian summermonsoon. Our results, combined with those for previously well-defined loess in the early Miocene, demonstrate activity(onset or possibly intensification) of an East Asian monsoon system around the Oligocene-Miocene boundary.