Abstract: Sesquioxidic soil clays from Oxisols in South Africa, Australia and Brazil, and two clays from Andosols in Japan and New Zealand, were investigated by selective dissolution techniques. Acid ammonium oxalate (pH 3) was found to be superior to currently popular alkaline reagents for extracting amorphous aluminosilicates and alumina from these clays. Boiling 0.5 N NaOH dissolved large amounts of finely-divided kaolinite and halloysite, while hot 5% Na2CO3 reaction was too slow (partial dissolution of synthetic amorphous aluminosilicates with one extraction) and insufficiently selective (gibbsite and kaolin of poor crystallinity dissolve to a variable extent). On the other hand, synthetic gels (molar SiO2/Al2O3 ranging from 0.91 to 2.55) dissolved completely after 2 hr shaking in the dark with 0.2 M acid ammonium oxalate (0.2 ml/mg). Specificity of oxalate for natural allophane was indicated by removal of similar quantities of silica and alumina using different clay:solution ratios. Oxalate extraction data indicated that allophane is absent in Oxisol clays. Allophane was determined quantitatively in volcanic-ash soil clays by allocating hydroxyl water content to oxalate-soluble silica plus alumina on the basis of an ignition weight loss-chemical composition function for synthetic amorphous alumino-silicates. Parameters of chemical reactivity and distribution of electric charges following various chemical pretreatments of allophane were found to correspond closely to those predicted on the basis of synthetic gel behaviour. Results for Oxisol clays suggested that the role of amorphous (oxalate-soluble) alumina in governing physicochemical properties is generally less than that of the poorly-crystalline, Al-substituted iron oxide component which is removed by deferration with citrate-dithionite-bicarbonate reagent.