A process for the preparation of propylene polymer compositions comprising from 50 to 90% by weight of a propylene (co)polymer fraction insoluble in xylene at 25° C., and from 10 to 50% by weight of an ethylene copolymer fraction soluble in xylene at 25° C., said process comprising: (i) a first step of polymerizing propylene in the optional presence of ethylene and/or C4-C10 alpha olefins, to produce a propylene (co)polymer being for at least 85% by weight insoluble in xylene at 25° C.; and (ii) a successive step, carried out in gas-phase, in the presence of the product coming from step (i), of copolymerizing a mixture of ethylene with one or more α-olefins CH2═CHR in which R is a hydrocarbon radical having 1-10 carbon atoms, to produce an ethylene copolymer; the process being carried out in the presence of a catalyst system comprising: the product obtained by contacting: (a) a solid catalyst component comprising a magnesium halide, a titanium compound having at least a Ti-halogen bond and at least two internal electron donor compounds one of which being present in an amount from 35 to 90% by mol with respect to the total amount of donors and being selected from succinates and the other being selected from 1,3-diethers, the total amount of internal electron donor compounds being lower than 14.0% by weight with respect to the total weight of the solid catalyst component; with (b) an aluminum hydrocarbyl compound, and optionally with (c) an external electron donor compound.