A process for foaming thermoplastic materials by adding to the plastics material 0.05 to 5 percent by weight of a dioxazolone of the formula I ##STR1## in which n is 1, 2 or 3, and when n is 1R is C.sub.1 -C.sub.12 -alkyl, C.sub.2 -C.sub.12 -alkenyl, C.sub.7 -C.sub.9 -aralkyl, C.sub.3 -C.sub.6 -cycloalkyl, C.sub.6 -C.sub.10 -aryl, or phenyl substituted by C.sub.1 -C.sub.9 -alkyl, halogen, C.sub.1 -C.sub.9 -alkoxy or nitro, or it is a heterocyclic radical, or when n is 2R is C.sub.1 -C.sub.8 -alkylene, ethenylene, m- or p-phenylene, m- or p-halophenylene, biphenylene, oxy-bis-(p-phenylene), 2,5-thienylene, 2,5-furylene, 2,6-pyridylene or naphthylene, or when n is 3R is phenenyl,and subsequently heating the mixture.