A method of surface treating particulate materials with electromagnetic radiation that is carried out with a so-called roll reactor comprising a rotating roll and an irradiation source. Radical former molecules can be applied on the surface of superabsorbent polymer particles, which are fed onto the surface of the roll and irradiated while moved with the rotating roll. The irradiation source is provided such that the radiation is able to reach at least part of the particulate material that has been fed onto the surface of the roll.