In a process for producing a tetrabromophthalic diester composition, a liquid reaction mixture is prepared comprising tetrabromophthalic anhydride (TBPA), a C2 to C6 polyhydric aliphatic alcohol (PAA) and an alkylene oxide (AO) selected from the group consisting of ethylene oxide and propylene oxide, said reaction mixture being substantially free of an organic solvent. While agitating the reaction mixture, the temperature of the reaction mixture is raised to at least 50° C. to allow the TBPA to react with the PAA and AO to produce a diester composition. The reaction is terminated when the diester composition has an acid value equal to or less than 0.25 mg KOH/gm of the diester composition.