Submicron iron oxide red, TiO2 and talc particles were chemically modified twice in the same double-cone reactor with vapor of 2,4,6,8-tetramethylcyclotetrasiloxane(TMCTS) and then 1-dodecene, 1-tetradecene or vinyl terminated poly(dimethylsiloxane). For the second modification alumina balls coated with hexachloroplatinic(IV) acid by simple impregnation of the catalyst-containing solution were used as a catalyst, reducing aggregation of the particles as well. The conversion of the second modification was even higher than that obtained in conventional liquid-phase hydrosilylation under comparable process conditions. Segregation of the particles from the ball was important for large volume modification. Under the given particle loading, the optimal ball size and catalyst concentration existed for the efficient use of catalyst. The ball catalysts were effective in four consecutive batches without any appreciable decrease in the conversion. Intermittent tumbling and the use of baffle enhanced the conversion of any gas-solid reaction such as both the first and the second modifications, minimizing simple tumbling (no mixing) and lump formation of particles. (C) 2003 Elsevier B.V. All rights reserved.