Process for the preparation of compounds of the formula ##STR1## wherein Hal represents chlorine, bromine or iodine and preferably chlorine, wherein R.sub.a represents hydrogen or a residue comprising one or more additional groups of the formula, ##STR2## from compounds ##STR3## wherein R.sub.2 represents hydrogen or a residue comprising one or more additional groups of the formula ##STR4## by reaction with gaseous hydrogen halide in the presence of a catalytic amount of an organic acid. Epoxy resins are prepared via the reaction of a compound of formula (A) in the presence of a basic compound.