The growth of Rhodobacter spharoides NCIMB8253 was investigated under photoanaerobic conditions with different concentrations of palm oil mill effluent in a dual effluent growth medium The influence of pulp and paper mill effluent as a alternative diluting agent on the bacterial growth was also investigated. During this study it was determined that the use of pulp and paper mill effluent as a diluting agent promoted bacterial growth compated with the use of raw palm oil mill effluent medium alone. The cell growth data for all experimental sets were well-fitted to the logistic and modified Gompertz models. The results revealed that 25% (v/v) palm oil mill effluent medium with pulp and paper mill effluent as a diluting agent namely G5 medium, can be applied as a suitable growth medium. A maximum colony-forming-units (CFU) number 360 x 10(8) CFU/mL was achieved in G5, which was higher that that for raw palm oil mill effluent and comparable with that for water diluted palm oil mill effluent medium. Furthermore chemical oxygen demand level (CODtotal and CODsoluble) reductions were observed in all treatments.