Abstract: The process for cleaving the heavy by-products derived from the synthesis of alkylchlorosilanes, in particular C.sub.1 to C.sub.6 alkylchlorosilanes, more particularly methylchlorosilanes, in particular for the cleavage of alkylchlorodisilanes, in particular methylchlorodisilanes having from one to five chlorine atoms, is carried out by treatment with hydrochloric acid in the presence of a catalytic system which comprises at least one metal phosphate, preferably associated with a basic impregnation compound.The metal phosphate may in particular correspond to the formula:M(XPO.sub.4).sub.yin which M represents a metal chosen from:monovalent metals, in which case X is 2H and y=1divalent metals, in which case X is H and y=1trivalent metals, in which case X is absent and y=1tetravalent metals, in which case X is: H and y=2pentavalent metals, in which case X is 0 and y=1.