The present application relates to a thermally expandable composition containing at least one peroxide cross-linking polymer, at least one peroxide and at least one endothermic, chemical blowing agent, said blowing agent comprising at least one solid, optionally functionalized, polycarboxylic acid or the salt thereof and at least one urea derivative according to the formula (I) as defined herein,to shaped bodies containing said composition and to a method for sealing and filling voids in components, for strengthening or reinforcing components, in particular hollow components, and for bondingmobile components using shaped bodies of this type.