Diatoms occupy a key branch in the evolutionary tree of oxygen-evolving photosynthetic organisms. Here, the electron transfer reaction mechanism from cytochrome c₆ to photosystem I from the diatom Phaeodactylum tricornutum has been analyzed by laser-flash absorption spectroscopy. Kinetic traces of photosystem I reduction fit to biphasic curves, the analysis of the observed rate constants indicating that electron transfer occurs in a cytochrome c₆/photosystem I transient complex, which undergoes a reorganization process from the initial encounter complex to the optimized final configuration. The mild ionic strength dependence of the rate constants makes evident the relatively weak electrostatically attractive nature of the interaction. Taken together, these results indicate that the "red" Phaeodactylum system is less efficient than "green" systems, both in the formation of the properly arranged (cytochrome c₆/photosystem I) complex and in the electron transfer itself. The results obtained from cross-reactions with cytochrome c₆ and photosystem I from cyanobacteria, green algae, and plants shed light on the different evolutionary pathway of the electron transfer to photosystem I in diatoms with regard to the way that it evolved in higher plants.