The separation of pseudocumene from the C₉ aromatic raffinate found in naphtha cracker was studied using adductive crystallization with thiourea. The charactristics of thiourea adduction are investigated on the bariables of feed ratio, temperature, solvent ratio and cooling rate, and optimum conditions on the separation of pseudocumene are investigated on these variables. Pseudocumene can be separated by four stages adductive crystallization from C₉ aromatics as a purity of 99.7wt%. With the trimethylbenzene isomers, the tendency for thiourea adduction is pseudocumene>hemimellitene>>mesitylene. The separation capacities of adductive crystallization for two binary mixtures of C₉ aromatics were compared with those of distillation and extractive crystallization. The extent of separation by one adduction is approximately 5.4 to 24.0 times greater than maximum separation that could be obtained by one theoretical distillation stage. The entrapping capacity is approximately 6.0times greater than the entrapping capacity that could be obtained by extractive crystallization.