Abstract Absorbance changes induced by flash illumination of a chloroplast suspension were monitored kinetically at selected wavelengths between 510 and 575 nm. Digital subtraction of the P518 component from the total transient signal permitted isolation of the responses due to cytochromes ƒ and b 6. The half rise time for cytochrome b 6 reduction (1.3±0.1 ms) was much greater than a previously reported value (< 10 μs); the half time for cytochrome b 6 oxidation was 35±4 ms. Cytochrome ƒ was oxidized with a half time of about 0.22 ms and the subsequent reduction occurred in two phases with half times of about 7.3 and 83 ms. These kinetic data show that cytochrome b 6 cannot be a primary electron acceptor in Photosystem 1. The rate of oxidation of cytochrome b 6 is consonant with this cytochrome being the source of electrons for the slower phase of cytochrome ƒ reduction.