The mass transfer rates of oxygen from air bubbles into both reverse osmosis (RO) and salt water (2.85% NaCl by mass) inside a baffled, well-mixed vessel have been investigated. Low aeration rates and impeller speeds were selected according to the Central Composite Rotatable Design (CCRD) experimental design approach. Empirical correlations with less than +/- 3% average prediction errors have been developed. Using identical operating conditions, the rate of oxygen transfer into pure RO water was significantly faster than that into salt water, with mass transfer coefficients ranging between 5.6 and 16.3 1/h for RO water and 4.4 and 14.7 1/h for salt water. The lower rates of oxygen transfer for salt water are hypothesized to be due to increased viscosity and correspondingly reduced diffusion in the boundary layer.