Electron transfer processes in cyanobacterial photosystem I particles fromSynechocystissp. PCC 6803 with a high potential naphthoquinone (2,3-dichloro-1,4-naphthoquinone) incorporated into the A1 binding site have been studied at 298 and 77 K using time-resolved visible and infrared difference spectroscopy. The high potential naphthoquinone inhibits electron transfer past A1, and biphasic P700+A1− radical pair recombination is observed. The two phases are assigned to P700+A1B− and P700+A1A− recombination. Analyses of the transient absorption changes indicate that the ratio of A- and B-branch electron transfer is 95:5 at 77 K and 77:23 at 298 K.