Preparation of bead-form expandable styrene polymers

A process for the preparation of bead-form, expandable styrene polymers by polymerizing styrene, in the presence or absence of further comonomers, in aqueous suspension in the presence of suspension stabilizers and conventional styrene-soluble polymerization catalysts and with addition of from 1 to 10% by weight of a C.sub.3 - to C.sub.7 -hydrocarbon and/or carbon dioxide as blowing agent and, optionally, conventional additives in effective amounts, which comprises using, as suspension stabilizer, a mixture of magnesium pyrophosphate, a sulfonate group-containing extender and an alkali metal or alkaline earth metal carboxylate, the magnesium pyrophosphate being prepared immediately before the polymerization by combining an aqueous solution of an alkali metal pyrophosphate with at least the stoichiometrically necessary amount of a magnesium salt.