The present invention concerns a method and apparatus for dehumidifying, drying or the like of different materials. The invention is developed primarily for dehumidification of sewage sludge (7), but it may be utilized for many different materials including foodstuffs as crispbread and pasta. The sludge (7) or other material is dehumidified or dried in a chamber (1) by means of thermal radiation. The thermal radiation is given by means of one or more elements (2) for thermal radiation. The thermal radiation is concentrated to one or more distinct wavelength ranges at which water has peaks for absorption of radiation energy. Air is circulated in the chamber (1), to take up moisture evaporated from the material.