Biological treatment of wastewater from a petrochemical complex was performed in order to evaluate the removal mechanisms of hydrocarbons by two parallel processes: rotating biological contactor (RBC) and activated sludge. The influent contained 700 mg/L COD, 140 mg/L BOD, 7.5 mg/L phenols and 32 mg/L hydrocarbons in emulsified form. The hydraulic detention time was 14 to 18 hours for activated sludge and 4 to 5 hours for RBC. It was found that both units produced an effluent containing a residual hydrocarbon of about 10 percent from the initial effluent concentration. The distribution of the removed hydrocarbon was: in activated sludge 70 percent by attachment to biosolids and wasted with the removed sludge; in RBC 15 percent were stripped to the atmosphere, 25 percent by biodegradation and 50 percent were removed by wasted sludge. The higher fraction of hydrocarbon biodegraded by the fixed film seems to be the result of higher active biomass which could concentrate in the RBC reactor. The fact that RBC removes less hydrocarbon to the atmosphere is considered as an environmental advantage. Although the fixed film attached a major part of the hydrocarbons, the biosludge production and characteristics for the fixed film were substantially better than for the suspended growth.