Food production in many parts of the developing world is constrained by acid soils that are low in plant-available phosphorus but high in toxic aluminum. Plant breeders are attempting to address these dual constraints by breeding crops with enhanced ability to exude organic acids from their roots. Enhanced root exudation of citrate, and perhaps malate, appears to detoxify Al while releasing P into the soil solution for plant uptake. Crops bred on one type of soil may respond very differently, however, when grown on soils with very different chemistry and mineralogy. The objective of this study is to characterize selected acid soils from Kenya and provide an interpretation of their properties for effective crop management. Eleven pedons representing three geomorphic provinces, (1) the Kericho and Kisii highlands west of the Rift Valley, (2) the Uasin Gishu Plateau and Lower level upland west of the Rift Valley (Eldoret, Kakamega, Vihiga, Siaya and Bumala), and (3) the volcanic foot ridges east of the Rift Valley (Embu, Kangema and Kerugoya), were described and sampled. Laboratory characterization so far consists of wet chemical analysis and x-ray diffraction (XRD). Exchangeable cations and extractable Al, cation exchange capacity (CEC) and effective CEC (ECEC) were determined by the unbuffered salt solution method. Powder mounts and a filter-membrane technique were used to prepare clay and fine silt fractions for XRD analyses. Soil pH increases with depth in all the pedons, but regional differences exist, with moderate acidity in the western highlands and strong acidity in the western plateau and the eastern volcanic foot ridges. Soils on the western plateau have lower organic C content compared to their counterparts. Although ECEC is approximately uniform with depth, there is regional variation. The CEC varies with depth and pedon, and ranges from 8.5 to 22 cmol_ckg^-1 in the topsoil. Soils east of the Rift Valley have significantly higher exchangeable Al than soils west of the Rift Valley. Pedons from the same geomorphic province tend to have similar mineralogical composition with no major differences in clay mineral distribution with depth. In the clay fraction, the 0.72 nm peak has the strongest intensity, indicating that kaolinite and/or halloysite are the dominant clay minerals. The widening of the 0.72 and 0.442-0.445-nm peaks around the base is a common phenomenon in most samples, strongly suggesting the presence of halloysite. For the soils west of the Rift Valley, a 1.0 nm peak indicates the presence of mica. Mixed layering of mica-kaolinite and/or mica–vermiculite in the fine and coarse clay is also apparent in most of the pedons west of the Rift valley, which is consistent with the ~2 times greater exchangeable K and higher CEC of soils in Kisii highlands.  Soils east of the Rift Valley contain considerably more gibbsite than their counterparts, consistent with their lower pH and higher Al saturation. We attribute the observed regional differences to the fact that soils in the eastern region developed from porous volcanic ash deposits associated with Mt. Kenya, while soils from the western region developed from less porous igneous rocks.