Using thoroughly dark-adapted thylakoids and an unmodulated Joliot-type oxygen electrode, the following results were obtained. (i) At high flash frequency (4 Hz), the oxygen yield at the fourth flash (Y 4) is lower compared to Y 3 than at lower flash frequency. At 4 Hz, the calculated S 0 concentration after thorough dark adaptation is found to approach zero, whereas at 0.5 Hz the apparent S 0 ( S 0 + S 1) ratio increases to about 0.2. This is explained by a relatively fast donation ( t 1 2 = 1.0–1.5 s ) of one electron by an electron donor to S 2 and S 3 in 15–25% of the Photosystem II reaction chains. The one-electron donor to S 2 and S 3 appears to be rereduced very slowly, and may be identical to the component that, after oxidation, gives rise to ESR signal II s. (ii) The probability for the fast one-electron donation to S 2 and S 3 has nearly been the same in triazine-resistant and triazine-susceptible thylakoids. However, most of the slow phase of the S 2 decay becomes 10-fold faster ( t 1 2 = 5–6 s ) in the triazine-resistant ones. In a small part of the Photosystem II reaction chains, the S 2 decay was extremely slow. The S 3 decay in the triazine-resistant thylakoids was not significantly different from that in triazine-susceptible thylakoids. This supports the hypothesis that S 2 is reduced mainly by Q − A, whereas S 3 is not. (iii) In the absence of CO 2/HCO − A and in the presence of formate, the fast one-electron donation to S 2 and S 3 does not occur. Addition of HCO − 3 restores the fast decay of part of S 2 and S 3 to almost the same extent as in control thylakoids. The slow phase of S 2 and S 3 decay is not influenced significantly by CO 2/HCO − 3. The chlorophyll a fluorescence decay kinetics in the presence of DCMU, however, monitoring the Q − A oxidation without interference of Q B, were 2.3-fold slower in the absence of CO 2/HCO − 3 than in its presence. (iv) An almost 3-fold decrease in decay rate of S 2 is observed upon lowering the pH from 7.6 to 6.0. The kinetics of chlorophyll a fluorescence decay in the presence of DCMU are slightly accelerated by a pH change from 7.6 to 6.0. This indicates that the equilibrium Q − A concentration after one flash is decreased (by about a factor of 4) upon changing the pH from 7.6 to 6.0. When direct or indirect protonation of Q − B is responsible for this shift of equilibrium Q − A concentration, these data would suggest that the p K a value for Q − B protonation is somewhat higher than 7.6, assuming that the protonated form of Q − B cannot reduce Q A.