This is the first detailed study of the micromorphology of archaeologically important cave sediments in the Great Cave of Niah, in the humid tropics of Sarawak, Borneo. Micromorphology is used to describe the sediments and post-depositional alteration, reconstruct the palaeoenvironments, and refine the environmental history of late Pleistocene deposits associated with the human remains (the so-called Deep Skull dated to ca. 43,000-42,000 B.P.). Micromorphology provides details of the shape, roundedness, arrangement, and chemistry of grains, aggregates, precipitates, and sedimentary structures that make up the cave sediments. The dominant processes in the West Mouth of the Great Cave of Niah are guano sedimentation, fluvial and shallow pond deposition interrupted by desiccation, mass movement, and chemical weathering. Also important is post-depositional alteration by bioturbation, mineral translocation and reprecipitation, and diagenesis. Micromorphology also provides evidence for short periods of soil development, burnt surfaces, and deposition of small fragments of bone within the sediment. Together this information indicates the fine details of the environment occupied by humans, the scale and effects of the mass movement processes that deformed the beds in which the human remains are preserved, and the taphonomic processes that reworked and redistributed archaeological material within this part of the cave.