We attempt to answer the question whether it is possible to directly compare the results of phosphorus uptake batch tests (PUBT) conducted by researchers who use different methods of determining the DPAOsto- PAOs ratio. This issue is raised because personally obtained results clearly show that DPAO fractions calculated with various methods proposed in the literature differ significantly. In the article the factors influencing the results of PUBT are discussed in detail. The study demonstrated that errors in determined values of DPAO fraction may be caused by the invitation of inappropriate value of reduction coefficient for anoxic conditions (η). The conclusion is that the η value should be accurately estimated in each test, independently for every active sludge.