We have measured the nanosecond kinetics of Chl-a + II reduction in oxygen-evolving detergent preparations of PS II from the cyanobacterium Phormidium laminosum and from higher plants (spinach) at 824 and 680 nm. Compared to earlier studies at 680 nm with higher plant material, we obtained an improved signal: noise ratio for measurements on a ns to ms time scale. The kinetics of Chl-a + II reduction in the ns range are consistent in the two preparations and are comparable to other studies of higher plant and cyanobacterial material. The ns kinetics are tightly connected to the ability for O 2 evolution. Analysis of the μs kinetics indicates three phases: (a) the slow phase ( t 1 2 ~ 150 μs in spinach and ~ 500 μs in Phormidium) reflects the back reaction between Chl-a + II and Q −; (b) the phase with t 1 2 5–10 μs is probably due to a donor which is not connected to an intact water oxidation system; (c) the intermediate μs component ( t 1 2 30–40 μs) may be related to water oxidation.