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Abstract Microsecond time-resolved step-scan FTIR difference spectroscopy was used to study photosystem I (PSI) from Thermosynechococcus vestitus BP-1 ( T. vestitus , formerly known as T. elongatus ) at 77 K. Photoaccumulated (P700 + – P700) FTIR difference spectra were also obtained for PSI from T. vestitus at both 77 and 293 K. The FTIR difference spectra are presented here for the first time. To greatly extend upon these FTIR studies nanosecond time-resolved infrared difference spectroscopy was also used to study PSI from T. vestitus at 293 K. Nanosecond infrared spectroscopy has never been used to study PSI samples at physiological temperatures, and here it is shown that such an approach has great value as it allows a direct probe of electron transfer down both branches in PSI. In PSI at 293 K, the infrared flash-induced absorption changes indicate electron transfer down the A- and B-branches is characterized by time constants of 33 and 364 ns, respectively, in good agreement with visible spectroscopy studies. These time constants are associated with forward electron transfer from A 1 – to F X on the B- and A-branches, respectively. At several infrared wavelengths flash-induced absorption changes at 293 K recover in tens to hundreds of milliseconds. The dominant decay phase is characterized by a lifetime of 128 ms. These millisecond changes are assigned to radical pair recombination reactions, with the changes being associated primarily with P700 + rereduction. This conclusion follows from the observation that the millisecond infrared spectrum is very similar to the photoaccumulated (P700 + – P700) FTIR difference spectrum.