A new two zone simplified oxygen mass-transfer model with an analytical solution was proposed and used to analyze the unsteady state reaeration data of diffused reaeration systems in order to find the volumetric mass-transfer coefficients. Series of unsteady state reaeration tests in a 1000 l tank was performed to validate the model. The effect of the air flow rate and the temperature upon the mass-transfer coefficients was studied and empirical equations were used to successfully correlate them. The air flow rate was varied from 0,50 to 1,33 - 10-3 normal m3/s and the water temperature was varied from 293,25 to 303,25 K. It was found that the new model describes mathematically the aeration process very well and gives a better physical understanding of it.