The bubble and surface volumetric mass transfer coefficients for oxygen, kLab and kLas, are separately determined for 179 aeration tests, with diffuser depths ranging from 225 to 32m, using the DeMoyer et al. (2003. Impact of bubble and free surface oxygen transfer on diffused aeration systems, Water Res 37, 1890-1904) mass transfer model. Two empirical characterization equations are developed for kLab and kLas, correlating the coefficients to air flow, Qa, diffuser depth, hd, cross-sectional area, Acs, and volumne, V. The characterization equations indicate that the bubble transfer coefficient, kLab, increases with increasing gas flow rate and depth, and decreases with increasing water volume. For fine bubble diffusers, kLab is approximately six times greater than kLab for coarse bubble diffusers. The surface transfer coefficient kLAx, increases with increasing gas flow rate and diffuser depth. The characterization equations make it possible to predict the gas transfer that will occur across bubble interfaces and across the free surface with bubble plume at depths up to 32m and with variable air discharge in deep tanks and reservoirs.