Mixtures of t,t conjugated linoleic acid methylester (t,t CLA-Me) isomers were prepared from synthetic CLA, consisting of 47.8% t10,c12 CLA; 45.5% c9,t11 CLA; 2.0% t,t CLA; and 4.7% others, by methylation with BF(3)/methanol (designated TT-TC/CT) in conjunction with purification at -68 degrees C for 24 h. The amount or composition of the TT-TC/CT was greatly affected by the concentration of BF(3) in methanol and the duration of methylation. The methylation of 50 mg of synthetic CLA for 30 min with 1 mL of 7.0% BF(3)/methanol produced a TT-TC/CT (21.54 mg) with the composition of 1.3% t12,t14; 5.9% t11,t13; 42.7% t10,t12; 44.0% t9,t11; 5.0% t8,t10; and 1.1% t 7,t9 CLA, whereas the methylation for 60 min with 14.0% BF(3)/methanol produced a TT-TC/CT (28.62 mg) with the composition of t,t CLA isomers different from that of TT-TC/CT by methylation for 30 min with 7.0% BF(3)/methanol. A large quantity of TT-TC/CT (14.15 g) with the composition similar to that of TT-TC/CT prepared from 50 mg of synthetic CLA was also prepared from 25 g of synthetic CLA. The purity of TT-TC/CT samples was greater than 98%. These results suggest that TT-TC/CT with a purity greater than 98% was easily prepared from synthetic CLA by BF(3)-catalyzed methylation, and the amount and composition of t,t CLA isomers of TT-TC/CT samples could be controlled by methylation conditions.