Sixteen profiles of alluvial soils of Vietnam were collected throughout the country and subjected to clay mineral analysis. Alluvial soils from different river systems/agro-ecological regions had different clay mineralogical compositions due to differences in sediment sources. Mica and kaolinite were found to be predominant clay minerals in alluvial soils. Other major minerals detected were vermiculite, smectite, chlorite, and the mica/vermiculite/smectite-mica/smectite mixed-layer mineral, depending on the river systems. The clay mineralogical composition was remarkably similar within soil units belonging to the same river system and in a profile. However, the clay mineralogical composition greatly varied with river systems; therefore, the regional variation existed and was mainly controlled by the mineralogical features of the sediment sources. Mica was found to be altered to mixed-layer minerals in some soils under specific conditions. The soil quality was assessed from the viewpoint of inherent potentiality based on the type and amount of clay minerals as plant-nutrient sources. Alluvial soils distributed in the Red River and Mekong River systems were evaluated to be in the high level of inherent potentiality. Those from the Trakhuc River and Ba River systems were classified to have low inherent potentiality, while those from the remaining ones were to be at an intermediate level.