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A 20-cm high column of MX80 bentonite compacted at dry density 1.70 g/cm3 with an initial water content of 16 percent was submitted to heating and hydration by opposite ends for 496 days (TH test). The temperature at the bottom of the column was set at 140??C and on top at 30??C, and deionised water was injected on top at a pressure of 0.01 MPa. Upon dismantling water content, dry density, mineralogy, specific surface area, cation exchange capacity, content of exchangeable cations, and concentration of soluble salts and pH of aqueous extracts were determined in different positions along the bentonite column. The pore water composition was modelled with a geochemical software. The test tried to simulate the conditions of an engineered barrier in a deep geological repository for high-level radioactive waste. The water intake and distribution of water content and dry density along the bentonite were conditioned by the thermal gradient. Liquid water did not penetrate into the column beyond the area in which the temperature was higher than 100??C. A convection cell was formed above this area, and liquid water loaded with ions evaporated towards cooler bentonite as it reached the area where the temperature was too high. In this area precipitation of mineral phases took place, Advection, interlayer exchange and dissolution/precipitation processes conditioned the composition of the pore water along the column. In most of the column the pore water was Na-SO4(2-) type, and changed to Na-Cl near the heater. TH treatment did not cause significant changes in the smectite content or the other mineral phases of the bentonite. Una probeta cil??ndrica de 20 cm de altura de bentonita MX-80 compactada a densidad seca 1,72 g/cm3 con humedad inicial del 16% se someti?? a calentamiento e hidrataci??n por extremos opuestos durante 496 d??as (ensayo TH). La temperatura en la parte inferior de la probeta se fij?? en 140??C y en la parte superior en 30??C, mientras se inyectaba agua desionizada por la parte superior. Al desmontar el ensayo se muestre?? la bentonita y se determinaron en diferentes posiciones a lo largo de la probeta su densidad seca, humedad, mineralog??a, superficie espec??fica, capacidad de cambio cati??nico, contenido de cationes intercambiables, y pH y concentraci??n de sales solubles en extractos acuosos. La composici??n qu??mica del agua de poro se modeliz?? con un c??digo geoqu??mico. Este ensayo simulaba las condiciones de la barrera de bentonita en un almacenamiento geol??gico profundo de residuos radiactivos de alta actividad. El gradiente t??rmico condicion?? la toma de agua y la distribuci??n de humedad y densidad a lo largo de la probeta. El agua l??quida no penetr?? en aquellas zonas de temperatura mayor de 100??C. Sobre esta zona se form?? una c??lula de convecci??n por la que el agua l??quida cargada con iones y transportada por advecci??n se evaporaba hacia zonas menos calientes. En esta zona se produjo la precipitaci??n de especies minerales. Los procesos de advecci??n, intercambio cati??nico y disoluci??n/precipitaci??n condicionaron la composici??n del agua de poro a lo largo de la probeta. En la mayor parte de ??sta el agua era tipo Na-SO4(2-) y cambi?? a tipo Na-Cl hacia el calentador. El tratamiento TH no provoc?? alteraciones significativas en la esmectita y las otras fases minerales.