This study describes a new technique for hydrophobisation of silica powders in a fluidised fluidised-bed reactor. The hydrophobisation treatment is based on silanisation reaction of silica with n-Octadecyltrichlorosilane (ODTCS). However, contrary to conventional silanisation processes, the reaction is performed at dry phase by mixing two classes of particles during the fluidisation: "target particles" to be treated (a fine microporous silica powder) and "ODTCS-carrier" particles consisting of coarser porous alumina beads containing an adequate amount of reagent. The results showed that the silanisation reagent was successfully anchored on silica surface and achieved high surface coverage (1.4 groups/nm(2)) in only few minutes. The energy consumption is reduced if the reaction is carried out at low temperatures but using humidified air (7% RH). These conditions promote however a vertical polymerization of the reagent on solids surface. However, an optimum temperature exists as, for example, the reaction at 80 degrees C with 7% RH gave a relatively interesting hydrophobic powder. (C) 2008 Elsevier B.V. All rights reserved.