Abstract: Internal standards for determination of 8 benzene homologues in waste water by GC-MS with static head-space sampling were optimized, based on the similarity of trends of responses between the analytes and the internal standards. As shown by the results, fluorobenzene and toluene-D8 were suitable as internal standards for benzene and toluene, respectively, and 4-bromo-fluorobenzene was suitable as internal standard for ethylbenzene, p-, m- and o-xylene, styrene and isopropylbenzene. As compared with using fluorobenzene as the only internal standard, accuracy and precision of the analysis were favourably improved by optimization of internal standards.