Separation of pseudocumene from the C-9 aromatic raffinates found in naphtha cracking by thiourea adduction was studied. For the trimethylbenzene isomers, the tendency for adduct formation with thiourea was found to be pseudocumene > hemimellitene /ggr/ mesitylene, and for the ethyltoluene isomers, p-ethyltoluene > o-ethyltoluene > m-ethyltoluene. From the eight cases of binary mixtures of C-9 aromatic compounds studied, separation by adduction with thiourea was more efficient than either distillation or extractive crystallization. Specifically, one adduction stage was approximately 5.4-58.0 times greater than one theoretical distillation stage. Similarly, the entrapping capacity by one adduction stage was approximately six times greater than that of extractive crystallization. Pseudocumene of purity of 99.7 wt% may be obtained from C-9 aromatic mixtures with the use of four thiourea adduction stages.