A frayed edge site can be a significant indicator of alterations in illitic minerals in soils evolving under different climates, because illitic minerals are known to enlarge the frayed edge site through vermiculitization. We investigated the relative abundance of this site in soil clays (< 2 mu m) using Radiocaesium Interception Potential (RIP) methodology, and determined the mineralogical factors controlling its amount (represented by RIP) for soils under ustic soil moisture regimes (northern Thailand, TH) and udic regimes (southwestern Japan, JP; East Kalimantan in Indonesia, ID) in humid Asia. We found three different correlations between RIP and mineralogical- or chemical-indices: positive (amount of illitic mineral and Cs-fixing capacity), negative (amount of kaolinite and hot-citrate extractable Al-Si) and no correlation (amount of DCB-oxides). The variable that correlated best with the logarithm of RIP in each region was Al(cit)-Si(cit) (JP), amount of illitic mineral (TH) and amount of kaolinite (ID). Under an ustic soil moisture regime in TH, there was little vermiculitization, as was evidenced by the direct correlation between the frayed edge site (or RIP) and the amount of illitic minerals and the very small Cs-fixing capacity. A distinct dry season restricts water leaching, which may reduce alteration in illitic minerals. For ID soils under an udic soil moisture regime, illitic minerals weather to form vermiculite, which is reflected in the enlargement of the frayed edge site. However, for JP soils, hydroxy-Al interlayering reduces the frayed edge site formed through vermiculitization. The release of hydroxy-Al polymers from vermiculitic layers can induce the re-exposure of the frayed edge site that can be progressive under a podzolization process.