Three Ultic Haplorthods with significant illuviation of clay and spodic materials in the subalpine forests located in the Alishan area of central Taiwan were selected to identify the clay mineral compositions by X-ray diffraction (XRD) and to explain the transformation of the clay minerals. The three pedons are dominated by vermiculite and vermiculite-illite interstratified minerals, and they have minor kaolinite, quartz and gibbsite. No hydroxy interlayered vermiculite (HIV) was found in the E horizon of the three pedons because the forest soils are very acidic and have very low Al contents. The presence of HIV in the spodic (Bhs) and argillic horizons (Bt) of the three pedons was associated with greater free Fe and Al contents (Fed and Ald), more favorable pH ranges, and coatings of organo-metallic complexes which prevented continuous weathering. The specific pedogenic process, clay illuviation and podzolization occurred sequentially in the Alishan area, and induced the unusual clay mineral distribution and transformation. The largest amounts of illite are in the C horizon and the amounts of vermiculite increased with decreasing soil depths. A reverse distribution between illite and vermiculite through the soil profile was observed. Illite was transformed to vermiculite due to the strong weathering environment associated with extremely low exchangeable K contents. The weathering sequence of clay minerals of Spodosols with fine textures in the study area is proposed as: illite →vermiculite (or interstratified vermiculite-illite minerals) →HIV and vermiculite.