DTA (differential thermal analysis) was used on mixtures of allophane and imogolite, and a calibration curve for estimating imogolite was obtained using the height of the endothermic peak near 400°C. This peak occurs between 390°C to 430°C, is distinctive and can be used to estimate imogolite in clay fractions and soils. An endothermic peak near 470°C observed in some Andisols probably arises from dehydroxylation of hydrated glass.