Abstract: An organic compound is bonded to a powdered solid support to produce, for example, a chromatographic stationary phase, by introducing to a bed of powdered material the vapor of a precursor of the compound to be bonded which precursor is selected from those which produce a gas phase by-product of the bonding reaction. The gas generated by the reaction is utilized as the fluidizing gas. This permits the use of fluidized bed techniques on extremely low particle size powders. One example is the reaction of alkylchlorosilanes with silica gel to produce stationary phases with bonded carbon chains, derived from the alkyl groups, of up to 24 carbon atoms. A second feature of the method is the hydrothermal pretreatment of the bed of powder with steam to precondition the support.