Photosystem II (PSII), the light-driven water/plastoquinone photo-oxidoreductase, is of central importance in the planetary energy cycle. The product of the reaction, plastohydroquinone (PQH2), is released into the membrane from the QB-site, where it is formed. A plastoquinone (PQ) from the membrane pool then binds into the QB-site. Despite their functional importance, the thermodynamic properties of the PQ in the QB-site, QB, in its different redox forms have received relatively little attention. Here we report the midpoint potentials (Em) of QB in PSII from Thermosynechococcus elongatus using EPR spectroscopy: Em QB/QB•−≈ 90 mV and Em QB•−/QBH2≈ 40 mV. These data allow the following conclusions: 1) the semiquinone, QB•−, is stabilized thermodynamically; 2) the resulting Em QB/QBH2 (~ 65 mV) is lower than the EmPQ/PQH2 (~117 mV), and the difference (ΔE ~50 meV) represents the driving force for QBH2 release into the pool; 3) PQ is ~ 50x more tightly bound than PQH2; 4) the difference between the Em QB/QB•− measured here and the Em QA/QA•− from the literature is ~234 meV, in principle corresponding to the driving force for electron transfer from QA•− to QB. The pH-dependence of the thermoluminescence associated with QB•− provided a functional estimate for this energy gap and gave a similar value (≥180 meV). These estimates are larger than the generally accepted value (~70 meV) and this is discussed. The energetics of QB in PSII are comparable to those in the homologous purple bacterial reaction center.