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In and lands, the soil surface commonly is characterized by textural and structural differentiation compared with deeper horizons. This is traditionally attributed to mechanical transfer of material by wind and runoff that contributes to sealing and crusting. Another process, however, was previously interpreted in the Niger Republic, at the surface of irrigated alluvial soils. The process, called xerolysis, is controlled by ultradesiccation under the Sahelian climate, and induces clay weathering and dissolution. The study areas are at the Sona station and the Tillakaina station where two irrigated perimeters extend on alluvial deposits of the Niger River. Fifteen soil profiles, eight at Sona and seven at Tillakaina, were sampled in and out the irrigated perimeters. The particle-size distribution was determinated by sedimentation and centrifugation methods. The clay fractions were studied by X-ray diffractometry, of which the first-order peaks were decomposed with a decomposition model, transmission electron microscopy, microdiffraction patterns from selected area electron diffraction, and energy dispersive X-ray analysis. The particles were clearly identified as beidellite, kaolinite, and mica. The decomposition analysis of the X-ray patterns from the < 0.1-mum clay subfraction indicated two populations of clays, one well crystallized in the surface horizons, and another weakly crystallized in the parent material. In the A horizons, the well crystallized smectite content decreased and its weakly crystallized correspondent increased. In the environmental conditions of Niger, xerolysis would be particularly efficient on the particles of weaky crystallized smectite sustaining weathering and total dissolution. (C) 2002 Elsevier Science B.V. All rights reserved.