Many sites are polluted by a large number of mixed contaminants. As mixed wastes, such petroleum/gasoline compounds as benzene, toluene, ethylbenzene, and three isomers of xylene (BTEX) and such chlorinated compounds as trichloroethylene (TCE) are among the most frequently found subsurface contaminants. Although many technologies have been developed for the remediation of contaminated sites, only a few of these technologies are applicable for mixed contaminated sites. Furthermore, even though lots of studies have been done with the removal of BTEX (singly and in mixtures), total petroleum hydrocarbons (TPH), or TCE (singly and in mixtures with other chlorinated aliphatic compounds; using such substrates as toluene) individually, unfortunately almost no study seems dealing with the interaction (stimulatory or inhibitory to each other’s removal) among BTEX, TPH, and TCE when they exist in such mixtures as BTEX/TPH and BTEX/TPH/TCE, especially under different environmental conditions. Even for the BTEX compounds alone, when BTEX were present in mixtures, the interaction results have been shown contradictory. vi Since knowing their detailed fates / behaviours during removal process is essential even before the remediation technology can be applied on site for the successful remediation of contaminated environment, BTEX and TPH as the representative petroleum hydrocarbons and TCE as the representative chlorinated hydrocarbon were tested in lab-scale under different conditions for concentration, pH and temperature in this study, to further elucidate the exact interaction during their bio-removal process when they co-exist in mixtures.