In this study, treatability of olive mill wastewater which had previously been treated physico-chemically together with domestic wastewater has been investigated using respirometric method. Olive mill wastewater (OMW) samples have been taken from a factory which produces olive oil with continuous method in Bursa. Acid cracking and physico-chemical treatability studies of the OMW have been carried out; a variety of coagulants with different dosages have been tested to achieve maximum removals of COD, SS and phenol. At 8000 mg/L alum concentration; 69% COD, 74% phenol and 87% SS whereas at 8000 mg/L ferric(III) chloride concentration; 80% COD, 79% phenol and 91% SS and whereas at 6000 mg/L Ca(OH)(2) concentration; 80% COD, 85% phe-nol and 86% SS removals were achieved. When ONW was mixed with activated sludge (SS = 3200 mg/L) at 1/1 (h/h), 1/2 (h/h) and 2/1 (h/h) ratios, decreases in respiration activity of 67%, 79% and 75% have respectively been observed. Among the three, only for the 1/2 (h/h) ratio of OMW/AS mixture in which OMW had previously been treated by lime, alum and ferric (III) chloride showed increases in respiration activities of 81%, 9% and 25%, respectively. Therefore, it can be said that OMW which was previously treated chemically with lime, alum and ferric (III) chloride, can be co-treated with domestic wastewater at a 1/2 (h/h) ratio. Among the three chemicals, lime could be preferred due to better removal efficiencies of COD, SS and phenol and lower cost.