Thermoplastic moulding compositions composed ofI) 99.8-95% by weight of(A) 0-100% by weight of one or more graft polymers comprise the monomers styrene, .alpha.-methyl styrene, methyl methacrylate or a mixture of 95-50% by weight of styrene, .alpha.-methyl styrene, ring substituted styrene, methyl methacrylate or mixtures thereof with 5-50% by weight of methacrylonitrile, acrylonitrile, maleric acid anhydride, N-substituted malerimides or mixtures thereof, grafted onto a rubber substrate, andB) 100 to 0%, by weight of one or more thermoplastic vinyl polymers, andII) 0.2 to 5% by weight of a polyalkylene ether modified by a radical former which can be compounds conventionally used as an initiator for radical polymerization such as peroxides.