Sulfonyltetrazoles of formula I, ##STR1## wherein n is 1 or 2, A is alkylene and R is a mono- or divalent hydrocarbon residue, are usable as blowing agents for thermoplastic polymers. These compounds decompose at temperatures above 200.degree. C, therefore they can be used preferably for foaming thermoplasts of higher softening temperature such as aromatic polycarbonates, polyesters or polyethers.