AbstractPhotosystem I (PS I) particles prepared from pea chloroplasts using either digitonin or Triton X‐100 as the solubilizing detergent were used to investigate the EPR characteristics of the electron acceptors A0, A1, and (Fe‐S)X. The signal from component A1appears asymmetric in both digitonin and Triton X‐100 solubilized samples. However, the difference spectrum which represents the isolated signal from component A0is symmetric in Triton X‐100 solution, but not in digitonin. Upon deuteration of digitonin solubilized PS I, the signal from A1became partially narrowed to give a spectrum comparable to that of deuterated Triton X‐100 solubilized particles. However, deuteration did not produce new spectral features in the digitonin solubilized preparation, but emphasized those already present Upon storage of either type of particles at 77 K, it was found that the signal from component A1, but not that from A0, decayed. The difference spectrum of A1obtained withg= 2.0058 and a peak‐to‐peak linewidth of 0.95 mT is compatible with the identification of A1as an anionic semiquinone. In digitonin solubilized particles, component A0and not A1affected the EPR signal of the iron‐sulphur center X, which suggests a magnetic interaction between A0and X. These results suggest that component A0is closer to X than is A1, and that A1may not be in a linear electron transport pathway between A0and the iron‐sulphur centers. They also show that Triton X‐100 perturbs the structure of