The bimolecular electron transfer from plastocyanin to P-700 + after flash excitation was investigated in inside-out vesicles from spinach chloroplast lamellae with randomly distributed photosystems and in vesicles from stroma membranes. Incubation of the vesicles with increasing amounts of Triton X-100 at room temperature revealed two opposing effects on the electron transfer rate. The maximum value of the rate constant which was 1.4 · 10 8 M −1 · s −1 in the absence of detergent, increased to the extremely high value of 5.3 · 10 8 M −1 · s −1 at 0.05% ( w v ) Triton X-100. However, at 1% ( w v ) Triton X-100 the rate constant dropped to the range of 10 5 M −1 · s −1. The effect of monovalent and divalent ions on the rate constant was used to characterize the electrostatic interaction between plastocyanin and the oxidizing site of Photosystem I under the following three conditions. (i) In intact membranes, the rate constant increased with increasing salt concentration as would be expected from the negative charges on both plastocyanin and Photosystem I. (ii) In the presence of 0.05% Triton X-100, the rate constant decreased with increasing salt concentrations, indicating two oppositely charged reactants. Extrapolation of the rate constant to infinite salt concentration gave a value of 2.5 · 10 8 M −1 · s −1 in the absence as well as in the presence of 0.05% Triton X-100. It is concluded that Triton X-100 solubilizes at this concentration a negatively charged component leaving a positively charged oxidizing site of Photosystem I, but does not change the local structure which is essential for efficient reaction with plastocyanin. (iii) In the presence of 1% Triton X-100, high cation concentrations (more than 0.5 M) were found to stimulate the severely inhibited rate constant to values of 10 6 M −1 · s −1. The effect of different cations indicates an about 10-times more negative surface charge density than in inside-out vesicles. The data are consistently described by a subunit which provides a positive charge at the highly negative oxidizing site on the Photosystem I reaction center protein, thereby facilitating the fast electron transfer from plastocyanin, and is removed by high Triton X-100 concentrations. The identity of the subunit and its occurrence in maize is discussed.