We studied the relationship between clay mineralogy and soil water extract composition to explain clay mineral distribution in humid Asia, namely Japan (JP), Thailand (TH), Java-Sumatra, Indonesia (JS-ID), and East Kalimantan, Indonesia (EK-ID) with relevance to soil forming factors, e.g. climate and parent material. Clay mineralogy was determined by XRD and DTA. Thermodynamic stability of clay minerals was evaluated with soil water extract composition. Presence of mica in parent material was necessary to vermiculite formation. The amount of 1.4 nm minerals was larger in JP and ID (Fig. a) where mica was unstable under lower pH condition (Fig. b). Mica was more stable under higher pH condition of TH. Gibbsite was generally found in JP and TH, of which the water extract composition was supersaturated with gibbsite (Fig. c). Its content was higher in JP where kaolin/gibbsite ratio correlated with H₄SiO₄⁰ activity (Fig. d). Hydroxy-Al interlayering between 2:1 layers was consistent with gibbsite occurrence. In conclusion, under higher pH conditions due to ustic moisture regime, mica was relatively stable and difficult to weather, while other primary minerals dissolved to form kaolinite. Under lower pH conditions due to udic moisture regime, mica weathered to form 1.4 nm minerals. In this case, JP soils are younger and had more amount of Al hydrolysis species in soil solution, resulting in formation of Al hydroxide between 2:1 layers and gibbsite. In JP gibbsite remained despite high H₄SiO₄⁰ activity, because of slow precipitation rate of the most stable mineral or kaolinite under lower temperature condition.