Clay mineralogy of soils derived from volcanic ash under tropical conditions was studied by differential dissolution, x-ray diffraction, DTA, infrared analysis, and scanning electron microscopy. Andepts and oxisols of the mountains and lowlands of Indonesia and Costa Rica were used. The results showed the presence of substantial amounts of amorphous material mixed with crystalline clay. The amorphous fraction in andepts, with SiO2/Al2O3ratios of 0.6, contained allophane as was indicated by scanning electron micrographs. Although the oxisols had amorphous substances with SiO2/Al2O3ratios of 1.0, which were in the range of allophane, scanning electron microscopy showed the presence of a dispersed and fine-grained compound different from allophane. The crystalline clay in the andepts and oxisols was identified by DTA and scanning electron analysis as kaolinite. X-ray diffraction did not reveal 1:1 lattice type of clay in andepts and exhibited only very weak peaks at 7.10–7.24 A in oxisols for halloysite and kaolinite. Data indicated that in Indonesia and Costa Rica gibbsite formation was greater in mountain than in lowland soils.