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Monitoring transmission changes at 820 nm, a measure of the redox states of plastocyanin (PC) and P700, is a good complementary technique for chlorophyll (chl) a fluorescence induction measurements. A thorough characterization of the properties of the 820-nm transmission kinetics during the first second after a dark-to-light transition is provided here for pea (Pisum sativum L.) leaves. The data indicate that plastocyanin in a dark-adapted leaf is in the reduced state. Three photosystem I (PSI)-related components, PC, P700 and ferredoxin, can contribute to the 820-nm transmission signal. The contribution of ferredoxin, however, is only approximately 5%, thus, it can be neglected for further analysis. Here, we show that by monitoring the sequential oxidation of PC and P700 during a far-red pulse and analysing the re-reduction kinetics it is possible to assign the three re-reduction components to PC (τ = 7–14 s) and …