Abstract Partial purification of the pigment complex responsible for light induced absorption changes at 700 mμ in photosynthesis was obtained. Removal from chloroplasts of water and hexane soluble materials, as well as 85% of the chlorophyll, left an active particulate preparation. Chlorophyll a probably is the pigment moiety of the complex which behaves as a single electron transfer oxidation-reduction system. Light minus dark difference spectra agreed with the oxidized minus reduced spectrum. Determination of the oxidation-reduction potential yielded at pH7 E′ 0 ≅ + 0.43 V.