Disclosed is a thermally expandable composition, comprising at least one polymer P, cross-linkable by peroxide, and between 1 wt.-% and 2.5 wt.-%, based on the total weight of the composition, of at least one acrylate A, and between 0.2 wt.-% and 2.5 wt.-%, based on the total weight of the composition, of at least one peroxide, and at least one blowing agent, characterised in that the equivalent ratio of said peroxide to said acrylate A is between 0.01 and 0.5, preferably between 0.13 and 0.41, and the weight ratio of said peroxide to said acrylate A is lower than 0.33 and said polymer P comprises or essentially consists of at least two polymers PI and P2, wherein PI exhibits a melt flow index (MFI) of between 100 and 200 g/10 min, and P2 exhibits a melt flow index of between 0.1 and 60 g/ 10 min, wherein MFI is determined by ASTM D1238. The thermally expandable composition shows excellent properties in terms of expansion stability and minimised buckling and is especially suitable for baffle and/or reinforcement elements, e.g. in automotive manufacturing.