A precoking selectivation technique is developed to improve the benzene purity in the heavy aromatics transalkylation over Pt/ZSM-12. The effect of precoking is selective to coking condition. Precoking in hydrogen can significantly improve benzene product purity. In contrast, precoking in helium results in severe deactivation of catalytic activity and deteriorates benzene product purity. For Pt/ZSM-12 at a platinum content of 200 pm as an example, subject to precoking treatment in hydrogen at a coke content of ca. 15.8 wt%, the benzene product purity can be improved up to the industrial specification of 99.85%. (C) 2007 Elsevier B.V. All rights reserved.