High assay, reaction-derived decabromodiphenylethane product is prepared by feeding (i) diphenylethane or (ii) partially brominated diphenylethane having an average bromine number less than about two, or (iii) both of (i) and (ii), into the liquid confines of a reaction mixture. Such reaction mixture is (a) formed from components comprised of excess liquid bromine and aluminum-based Lewis acid bromination catalyst, and (b) maintained at one or more elevated reaction temperatures of from about 45°-90° C., and at least when elevated pressure is needed to keep a liquid state in the reaction mixture at the temperature(s) used, the reaction mixture is at such an elevated pressure, whereby ar-bromination occurs. The feeding is conducted at a rate slow enough to form high assay reaction-derived decabromodiphenylethane product, which is an effective flame retardant.