The present study focused on the influence of the apparent viscosity of activated sludge on mixed liquor oxygen transfer rates, and examined whether both could be modified by the addition of an organic polyelectrolyte. Batch experiments carried out using mixed liquor samples in a stirred tank reactor revealed that alpha-factors, which incorporate any difference in oxygen mass transfer coefficient values between clean water and mixed liquor samples, showed exponential relationships when plotted against increases in mixed liquor apparent viscosities and sludge mixed liquid suspended solids values. Furthermore, oxygen mass transfer coefficients increased exponentially with increases in the Reynolds number, a parameter which reflects liquid turbulence levels. Oxygen mass transfer rates of five sludge samples collected from four full scale wastewater treatment plants (WWTPs) also increased by about 10% after flocculation, while with a sludge sample collected from the lab scale reactor, it decreased by about 5%. One likely explanation is that the apparent viscosities of the WWTPs and the lab-reactor sludges decreased and increased respectively after flocculation. Such changes probably reflect changes in the volume fraction of mixed liquor suspended solids as indicated by changes in sludge volume indices, particle size values and endogenous respiration rates before and after flocculation. These findings assist in developing an understanding of how oxygen mass transfer characteristics may be affected by activated sludge suspensions, and hence assist in reducing the operational costs of WWTPs. (c) 2013 Elsevier B.V. All rights reserved.