The S-state distribution in a dark-adapted photosystem II preparation, isolated from spinach chloroplasts, can be changed by pH. The period 4 oscillation of the oxygen-releasing reaction, as measured by UV absorbance changes, is shifted by one flash at pH 8.3. This is not due to the generation of the EPR signal II on the first flash, or to the loss of a positive charge to some other electron donor after the first flashes. It is concluded that at pH 8.3 the S1-state is reduced to S0. The half-time of the reduction of S1 to S0 is 45 s, and the reduction is reversed in the dark if the pH is readjusted to pH 6.0. The oxygen release at pH 8.3 is irreversibly slowed down to about 3.2 ms.
