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Biological conversion of syngas (CO, H2 and CO2) has gained a widespread interest in the recent decades. The anaerobic microorgansisms called as “acetogens” utilize CO and CO2/H2 as carbon and energy sources, and produce useful fuels and chemicals. However, the low solubility of CO and its high affinity to metal ions in the CO oxidizing enzymes are big hurdles to achieve a suscessful operation of commercial sized bioreactor. It has been already reported that high concentration of CO during reaction reduces the activity of CO oxidizing enzymes and hence inhibits the microbial growth. Thus the optimum concentration of the CO in the fermentation media is desired to design and control large scale fermenters. The aim of this sudy is to evaluate the CO utilizing capacity for a newly isolated strain which is identified as Oscillibacter sp. cow-5. This strain is a strict anerobic acetogen which produces various chemicals: acetate, butyrate and small amount of ethanol and iso-valerate from CO. The strain was grown under optimum growth conditions in a batch reactor (serum vials) and a modified Monod model equation was used to determine the maxmium growth rate (μmax) and Monod constant (Ks). Simulated data were well fitted with experimental one, and two paramters, μmax and Ks, were estimated as 0.05 h-1 and 0.044 mM CO, respectivley using non linear second order regression. Meantime, the apparent dissolved CO inhibition has been found at 0.8 mM CO (equivalent to CO solubility under 1atm of PCO) from the plotting of C*vs C*/μ.