Morphological studies of some of the dark clay soils of Puerto Rico indicate that they are similar to the black or dark soils of the Tropics and of other areas now mostly classified as Vertisols and some as Mollisols. They are clayey with high saturation and porosity. Small pores predominate, however, resulting in low hydraulic conductivity and poor internal drainage. They are alkaline, soil pH increasing with depth. Organic matter is generally high in the surface horizons. Quartz is the dominant mineral in the silt fraction, but lesser amounts of feldspars are present in all the soils. Montmorillonite is the dominant clay mineral with kaolinite and quartz occurring in trace to small amounts. Montmorillonite and kaolinite, in some profiles, showed a greater degree of crystallinity by their more symmetrical diffraction peaks. X-ray diffraction results for the Aguirre and Guánica soils were indicative of a minimum amount of weathering. The clay minerals in the surface horizon of the Ponceña and Camagüey soils showed a small amount of weathering. The data available indicate that the dark clay soils of Puerto Rico meet many of the requirements of the Pellusterts, Pelluderts, and Rendall soil groups.