The present invention relates to a process for the preparation of 3-chloropropyldimethylchlorosilane by hydrosilylation reaction in a reaction medium comprising dimethylhydrochlorosilane and allyl chloride, in the presence of a catalytically effective amount of di-μ-chlorobis(η-1,5-cyclooctadiene)diiridium, the said process being characterized in that at least one auxiliary in the free or supported state selected from the group of compounds consisting of: (i) ketones, (ii) ethers, (iii) quinones, (iv) anhydrides, (v) unsaturated hydrocarbon compounds (UHC) having an aromatic nature and/or comprising at least one C═C double bond and/or at least one C≡C triple bond, it being possible for these unsaturated bonds to be conjugated or nonconjugated, the said UHCs being linear or cyclic (mono- or polycyclic), having from 4 to 30 carbon atoms, having from 1 to 8 ethylenic and/or acetylenic unsaturations and optionally comprising one or more heteroatoms, (vi) and their mixtures, is added to the reaction medium, with the condition according to which, when the auxiliary comprises one or more UHCs as defined above, then this (these) UHC(s) is (are) mixed with at least one other auxiliary other than a UHC.