A silane or siloxane containing at least one organic group is prepared by a hydrosilylation reaction between a silane or siloxane (A) containing at least one Si--H group and an allyl compound (B) in the presence of a noble metal catalyst. The silane or siloxane (A) and the allyl compound (B) are fed continuously through a reaction zone which is maintained at a temperature in the range 115 200.degree. C. and which is provided with heat exchanger means to dissipate the heat from the exothermic hydrosilylation reaction. The residence time of (A) and (B) in the reaction zone is less than 20 minutes.