More than half of the total number of soils examined belonged to 7Å mineral dominant type in clay mineral composition. However, some distinct characteristics were recognized among different soil groups and different regions.
　Marine Alluvial Soils in the Central Plain were composed of mica clay minerals, kaolinite minerals and 14Å minerals, consisted mainly of montmorillonite, in a nearly equal quantity. Brackish Water Alluvial Soils were not different from Marine Alluvial Soils in their clay mineral composition, except for the presence of a small amount of chlorites.
　The clay mineral composition of Fresh Water Alluvial Soils and Low Humic Soils varied markedly with the regions. For example, various types of clay mineral composition ranging from 7Å to 14Å mineral-dominant type were found in the Central Plain, and their 14Å minerals consisted mainly of montmorillonite and vermiculite, dipending on the nature of the parent materials and the weathering history of the soils. Soils from the North-eastern Region were characterized by a high content of kaolinite minerals and the absence or presence of a small amount of mica caly minerals, and their 14Å minerals were mainly Al-interlayered ones. But, the soils distributed along the Mekong River were dominanted by mica clay minerals, and chorite was the main component of their 14Å minerals. Soils from the Northern Region were either of the 7Å or 10Å mineral-dominant types. Clay mineral species composed of 14Å minerals were mainly vermiculite and partly montmorillonite. Soils from the Southern Region were more kaolinitic than those of the other three regions. A low content of 14Å minerals was a feature probably common to soils in this region, except for Marine Alluvial Soils and Brackish Water Alluvial Soils.
　Humic Gley Soils locally distributed in Chiang Rai Basin were similar in clay mineral composition to other soil groups found in this basin. (Hydromorphic) Non-Calcic Brown Soils were either of the 7Å or 14Å mineral dominant types, and their 14Å minerals consisted mainly of vermiculite and partly of chlorite in the Central Plain, while mainly of Al-interlayered minerals in the North-eastern Region. Grumusols derived from marly sediments, which are locally distributed in Lop Buri in the Central Plain, were highly montmorillonitic in their clay mineral composition.