The invention concerns the preparation of halogenated monoorganoxysilanes, of formula (I), said compounds being useful as synthesis intermediate in organic chemistry. Said method for preparing monoorganoxysilanes consists in: using as starting product halogenoalkylsilanes of the (CH.sub.3).sub.2SiCl.sub.2 type and in substituting the silicon with a radical bearing a divalent unit bound to an electrophilic reactive group capable of reacting with at least an appropriate nucleophilic agent to form a functionalised monoorganoxysilane of formula (II) with, for example: R<1>=C.sub.1-C.sub.4 alkyl; R<2>, R<3>=C.sub.1-C.sub.6 alkyl; B=C.sub.1-C.sub.10 alkylene; m=1 or 2; Hal=halogen; W=amino, mercapto, (organosilyl)-organopolythio radical.