The reduction of derivatives of the type RnSiσ4-n (σ = Cl, OCH3) by some organomagnesium compounds RMgX at high temperature (⩾80°) leads to the hydrosilanes RR'SiH2 and RR'2SiH in various ratios according to the considered reagents. The results described in this publication constitute a first approach in the knowledge of the mechanism of these reductions. A deeper study of the mechanism is the subject