A thermally expandable composition comprising a thermoplastic polymer compound and/or an elastomer compound, at least one blowing agent including at least one dicarboxylic acid salt of an aminoguanidine compound, optionally at least one free radical initiator and/or a vulcanization system, and at least one guanidine derivative. The invention is also related to baffle and/or a reinforcement element for hollow structures including the thermally expandable composition, to a process for manufacturing the baffle and/or reinforcement element, to use of the baffle and/or reinforcement element for sealing, baffling, or reinforcing of a cavity or a hollow structure, and to a method for sealing, baffling and/or reinforcing a cavity or hollow structure.