Four areas of red soils under tropical monsoonal climate in Thailand were selected for the study of soil genetic implication and mineral properties to interpret environmental conditions of Thai red Oxisols. The method of study included pedon analysis of soils in the field and laboratory analysis following standard methods. Results of the study revealed that all of them can be classified taxonomically as Typic Kandiudox and Rhodic Kandiudox. They developed on undulating plain under very high rainfall regime of the tropical monsoonal climate that supports the practice of tropical orchards in Southeast coast and Peninsular regions of Thailand. Two of them are on lava corrosion plain of basalt and the others are on karst corrosion plain of limestone. The clear illuviation and ferrugination are manifested by the kandic horizon (Bto) and dominance of free iron oxides with very low amounts of cations except for potassium. The rather unique clay assemblage of kaolins, hematite, goethite, hydroxy-interlayered vermiculite (HIV), and gibbsite in soils developed on basalt, and the horizontal and vertical uniformity are clear indicators of intensive and fast weathering. The fast and deep weathering results from a combination of an easily weathered parent rock exposed to a hot and prolonged moist pedoclimate. This combination and the resulting soil development are reflected in the observed morphologies of purified kaolin and iron oxide minerals. Kaolins in soils developed on basalt have very small size (9-12 nm) and consisting of anhedral plates, and crystal sizes of goethite and hematite are also very small. These crystal sizes tend to negatively relate to the extent of Al substitution in these minerals. Soils developed on limestone have a larger kaolin crystal size (17-20 nm) with higher proportion of euhedral platy crystals, and crystal sizes of their goethite and hematite are distinctly larger than those derived from basalt. Their mineral properties relate well with their difference in specific surface area (SSA) and cation exchange capacity (CEC). These properties have direct influence on soil fertility. Small crystals of kaolin and iron oxide minerals provide high SSA and CEC. The substantial cation exchange capacity of kaolin in these soils is important for buffering and nutrient retention particularly in soils which have little organic matter. Iron oxide properties strongly affect anion retention.