Petroleum-based products and anthropogenic organic compounds have been continuously contaminating soil and groundwater ever since their usage began. As mixed wastes, such petroleum/gasoline compounds as benzene, toluene, ethylbenzene, and three isomers of xylene (BTEX) together with such chlorinated compounds as trichloroethylene (TCE) are among the most frequently found subsurface contaminants. Both BTEX and TCE are carcinogenic and would cause other serious health effects on humans. 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 to have dealt 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, BTEX/TPH/TCE, and BTEX/TCE, especially under different environmental conditions. vii Seeing that knowing their detailed fates/behaviors during bioremoval 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 experiments under different conditions of contaminations concentration, pH, temperature, and amount of dissolved oxygen in this study. In the meantime, a central composite design (CCD) technique was also used to develop mathematical equations, in terms of pollutant removal from contaminated water, hoping giving another angle of view to further elucidate the exact interaction during their bioremoval process when they co-exist in mixtures.