"Anaerobic Stabilization" (AnS) is a phenomenon previously observed in biological nutrient removal (BNR) systems that use anaerobic-aerobic sequencing for phosphorus and/or nitrogen removal. AnS manifests itself in the form of less-than-theoretical oxygen requirements for the extent of organics stabilization observed. The objectives of this study were to develop an improved methodology for the quantitative determination of AnS, verify the occurrence and validate the statistical significance of AnS, identify components of the AnS-related redox balance, and investigate possible explanations for AnS. A lab-scale continuous-flow A/O¹ system was operated with chemical inhibition of nitrification at a 12-hour nominal HRT, 10-day BSRT (Biological Solids Retention Time), 1Q RAS flow, and varying synthetic feed compositions. Data from this system were used to demonstrate that, by eliminating the need to quantify the clarifier OUR, the Boundary Exchange AnS determination method developed in this study afforded a major advantage over earlier methods. Non-zero AnS was shown to be a statistically significant, reproducible phenomenon. Carbon, oxygen, and sulfur were identified as the three main elements affecting AnS in the A/O system studied. A second lab-scale A2/O system operating at a 6-hour nominal HRT, 5-day BSRT, 1Q RAS flow, and 2Q RNX flow, and receiving raw municipal wastewater feed spiked with acetate, was uSed in conjunction with the A/O system to study possible AnS explanations. A combination of processes accounted for varying percentages of observed AnS. Hydrogen production explained 0.1 percent or less, while methane production explained almost 19 percent with formate in the feed but no more than 0.8 percent without it. Aeration-induced Stripping of reduced volatiles explained up to 6 percent. Attempts to identify the reduced volatiles revealed traces of ethanol but no n-butanol in the A2/O system. Limitations of the COD test were identified as a possible explanation for AnS that warrants further investigation. A unified speculative biochemical model consistent with all results of this study and with established theory, and capable of partially explaining observed AnS, is proposed in this study. ¹A/O and A2/O are trademarks of Air Products and Chemicals, Inc., Allentown, PA, U.S.A. / Ph. D.