Photosystem II (PSII) is a multisubunit enzyme embedded in the lipid environment ofthe thylakoid membranes of plants, algae and cyanobacteria. Our recent chlorophyll- afluorescence transient measurements, using single-turnover saturating flashes (STSFs), revealedthe involvement of conformational changes in the dark-to-light transition of PSIIand a previously unknown rate-limiting step (1). In diuron-inhibited PSII, capable of onlyone stable charge separation, the fluorescence maximum could only be reached by a trainof STSFs and with sizeable waiting times ( ) between consecutive flashes. In PSII corecomplexes from T. vulcanus , a minimum of 1 ms was found at 20 C. In this work, weperformed nanosecond transient absorption spectroscopy measurements (2,3), detectingat 819 nm the photooxidation and re-reduction kinetics of the primary electron donor P680 in diuron-treated PSII core complexes. We used double laser-flash excitations withvariable between the flashes. The experiments showed a rapid (4 ns) recombination of P680 + Phe - upon the second flash for all values tested. These data offer no explanationon the origin of rate-limiting steps in PSII. Hence, the dark-to-light transition of PSIImost probably originates from light-induced reorganizations, e.g. dielectric relaxations,affecting the fluorescence yield of chlorophyll-a.