Halogen-containing s-triazine compounds of the formula I ##STR1## where R is OH, R.sup.1, R.sup.2 or R.sup.3, R.sup.1 is ##STR2## where R.sup.4 is lower alkyl or lower haloalkyl, X is bromine or chlorine, k and n are 0 or integers from 1 to 5 and k+n is not more than 5, R.sup.2 is ##STR3## where R.sup.5 is lower alkyl or lower haloalkyl, Z is bromine or chlorine, s and p are 0 or integers from 1 to 4 and s+p is not more than 4, and R.sup.3 is ##STR4## where R, R.sup.5, Z, s and p have the above meanings. The compounds claimed are prepared by reacting cyanuric chloride with a phenol of the formula III ##STR5## and a benzene derivative of the formula IV ##STR6## in the presence of a base. The products thereby obtained are used for flameproofing thermoplastics, in particular styrene/acrylonitrile copolymers made impact-resistant with rubber (ABS resins).