The oxygen mass transfer kLa is generally studied under non-reactive conditions, leaving out the most fundamental operational condition in activated sludge processes (ASPs). Existing oxygen transfer models, used in wastewater treatment plant design and optimizations, have therefore a major shortcoming. More accurate kLa models lead to improved system analysis and knowledge. This work studied the volumetric oxygen mass transfer kLa in an ASP, under varying operational conditions. An empirical correlation for kLa versus nine studied variables (tank volume (Vt), height (Ht), diameter (Dt), surface area (At), airflow rate (Qa), diffusers surface area (Ad) and depth (hd), bubble size (db) and dynamic viscosity (l)) for clean wate (kLaCW) and for activated sludge (kLaAS) in a diffused aerated cylindrical batch reactor is created. The experimental results were used to develop a high fit empirical model for kLaAS (R2 = 0.96) and kLaCW (R2 = 0.95). The following equations were obtained (kLa in s_1):