Biologically enhanced oxygen transfer has been a hypothesis to explain observed oxygen transfer rates in activated sludge systems that were well above that predicted from aerator clean-water testing. The enhanced oxygen transfer rates were based on tests using BOD bottle oxygen uptake rates (OURs) on samples removed from the activated sludge system. Bench- and full-scale plant studies were performed to compare results of in situ OUR measurement techniques to BOD bottle OUR measurements. The studies showed that the condition of low dissolved oxygen (DO) and high OUR for which the greatest enhanced oxygen transfer was reported resulted in OURs in the BOD bottle tests well above the actual in situ OURs. For high DO and high OUR activated sludge operating conditions, the BOD bottle OURs were below the actual in situ OURs. The BOD bottle OUR values accurately described in situ OURs for endogenous respiration conditions with nonlimiting DO concentrations. The results suggest that previously observed biologically enhanced oxygen transfer was not actually occurring but was the result of the BOD bottle test method and activated sludge operating condition.