Liquid-phase cycloisomerization of 5-(o-tolyl)-pentene (OTP) was conducted using a commercial nanocrystalline zeolite BEA. Zeolite BEA was a good catalyst for selective synthesis of 1,5-dimethyltetralin (1,5-DMT). However, over certain zeolite BEA, isomerization of 1,5-DMT to other isomers occurred after cyclization of OTP was almost complete at high temperatures. Textual properties of nanocrystalline BEA did not influence the cyclization activity, but the degree of postsynthetic treatments such as surface poisoning, passivation, and dealumination had great effects. Reactions over BEA modified by triphenylphosphine poisoning and Si layer coating of the outer surface of zeolite crystatlites revealed that strong acid sites residing only on the external surface of zeolite BEA were responsible for isomerization activity. The Al-27 magic-angle spinning nuclear magnetic resonance study for dealuminated zeolites together with the IR study of adsorbed pyridine clearly demonstrated that the concentration of Bronsted acid sites due to the framework aluminum was directly related to cyclization activity. On the other hand, octahedral aluminum did not participate in either OTP cyclization or DMT isomerization.