We studied the forms, properties, and significance of active Al and Fe in the humus horizons of Andosols from northeastern Japan and formed a hypothesis to elucidate the surface weathering of volcanic ash. Our soil samples were divided into two groups: nonallophanic and allophanic. Nonallophanic soils showed that Al of weathering products is present in major amounts in 2:1 layer silicates and in minor amounts in Al-humus complex. The Fe of weathering products, however, consisted mainly of the “free”form, which is largely complexed with humus. A small portion of this element was contained in the layer silicates. The Al of weathering products in allophanic soils was found in large amounts in allophane and imogolite and in small amounts in allophanelike constituents and Al-humus. The Fe of weathering products in these soils consisted wholly of “free” Fe, which is partly complexed with humus. We indicated that the formation of Al- and Fe-humus complexes, allophane, and imogolite is governed by soil acidity to a great extent. Formation of Al- and Fe-humus complexes proceeded with decreasing pH(H2O). The formation of allophane and imogolite took place in the pH(H2O) >4.9, however, and had an inverse relationship with formation of the Al-humus complex. The nonallophanic and allophanic soils, with a few exceptions, showed pH (NaF) values >9.4 and large phosphate absorption. We noted that the active components of Andosols show a wide variation in their contribution to unique chemical properties of the soils. In order to bring nonallophanic Andosols into Andepts, we emphasized the necessity of redefinition of “amorphous material.” Finally, based on our present and previous studies, we showed a new weathering scheme for volcanic ash in modern humus horizons.