Bromine-containing phosphonic acid esters of the formula ##EQU1## in which R represents a halogen-containing linear or branched aliphatic or cycloaliphatic hydrocarbon radical having from 1 to 8 carbon atoms and R' and R" are hydrogen, halogen or alkyl groups having from 1 to 4 carbon atoms which may be closed to form a ring and/or substituted by halogen, at least one substituent being a bromine atom, are suitable as flame retardants for plastic materials, especially polystyrene and styrene copolymers.