Exposure of isolated photosystem I (PSI) complexes to illumination (2300 μE m⁻² s⁻¹) for various periods of time resulted in striking changes in their absorption spectra. A 6 nm blueshift of the absorption maximum in the red was detected after 100 min illumination. The fourth derivative of the absorption spectra verifies that the main change of the red peak was attributed to the 682 nm absorption band. Further, it was also shown that a shoulder in the absorption spectra located around 470 nm decreased after the first 5 min of illumination and almost disappeared after 40 min illumination, suggesting that chlorophyll b bound to light-harvesting complex I (LHCI) is also sensitive to excess light. A maximum inhibitory effect on the oxygen uptake rates and a strong stimulation were observed when the PSI complexes were exposed to illumination for about 20 and 40 min, respectively. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis shows that LHCI-680 started to degrade during the first 5 min of illumination and almost completely disappeared after 40 min of illumination. These observations demonstrated that LHCI was more sensitive to illumination than the PsaA/B subunits which also presented some degradation signs after 40 min illumination. In addition, insoluble–cohesive-denatured proteins also appeared between the stacking and resolving gel after prolonged illumination (100 min). A photoprotective function of LHCI for the PSI reaction center is proposed.